Maybe someone can help me. I have created a custom module to start a migration of a csv file based on the code from the migration source ui module. Is it possible to update a field default value with data from my form submit ? I tried $migration->setProcessOfProperty($fieldname, $config) but cant get it to work. My Code looks like this:

class ExampleForm extends FormBase {    /**    * The migration plugin manager.    *    * @var DrupalmigratePluginMigrationPluginManager    */   protected $pluginManagerMigration;    /**    * The migration definitions.    *    * @var array    */   protected $definitions;    /**    * Config object for migrate_source_ui.settings.    *    * @var DrupalCoreConfigImmutableConfig    */   protected $config;    private $addFieldMapping;    /**    * MigrateSourceUiForm constructor.    *    * @param DrupalmigratePluginMigrationPluginManager $plugin_manager_migration    *   The migration plugin manager.    * @param DrupalCoreConfigConfigFactoryInterface $config_factory    *   The config factory service.    */   public function __construct(MigrationPluginManager $plugin_manager_migration, ConfigFactoryInterface $config_factory) {     $this->pluginManagerMigration = $plugin_manager_migration;     $this->definitions = $this->pluginManagerMigration->getDefinitions();     $this->config = $config_factory->get('migrate_source_ui.settings');   }    /**    * {@inheritdoc}    */   public static function create(ContainerInterface $container) {     return new static(       $container->get('plugin.manager.migration'),       $container->get('config.factory')     );   }    /**    * {@inheritdoc}    */   public function getFormId() {     return 'confidence_migrate_example';   }    /**    * {@inheritdoc}    */   public function buildForm(array $form, FormStateInterface $form_state) {     $options_verfahren = [];     $options_verwalter = [];     $verwalter_ids = Drupal::entityQuery('user')       ->condition('status', 1)       ->condition('roles', 'kunde')       ->execute();     $verfahren = Drupal::entityTypeManager()       ->getStorage('node')       ->loadByProperties(['type' => 'verfahren']);     $verwalter = Drupal::entityTypeManager()       ->getStorage('user')       ->loadMultiple($verwalter_ids);       foreach ($verfahren as $node) {       $options_verfahren[$node->id()] = $node->label();     }      foreach ($verwalter as $user) {       $options_verwalter[$user->id()] = $user->label();     }     $options = [];     foreach ($this->definitions as $definition) {       $migrationInstance = $this->pluginManagerMigration->createStubMigration($definition);       $aa3 = $migrationInstance;       if ($migrationInstance->getSourcePlugin() instanceof CSV || $migrationInstance->getSourcePlugin() instanceof Json || $migrationInstance->getSourcePlugin() instanceof Xml) {         $id = $definition['id'];         $options[$id] = $this->t('%id (supports %file_type)', [           '%id' => $definition['label'] ?? $id,           '%file_type' => $this->getFileExtensionSupported($migrationInstance),         ]);       }     }     $form['migrations'] = [       '#type' => 'select',       '#title' => $this->t('Migrations'),       '#options' => $options,     ];      $form['verfahren'] = [       '#type' => 'select',       '#title' => t('Verfahren'),       '#options' => $options_verfahren,     ];      $form['verwalter'] = [       '#type' => 'select',       '#title' => t('Verwalter'),       '#options' => $options_verwalter,     ];      $form['source_file'] = [       '#type' => 'file',       '#title' => $this->t('Upload the source file'),     ];     $form['update_existing_records'] = [       '#type' => 'checkbox',       '#title' => $this->t('Update existing records'),       '#default_value' => 1,     ];     $form['import'] = [       '#type' => 'submit',       '#value' => $this->t('Importieren'),     ];     return $form;   }    public function validateForm(array &$form, FormStateInterface $form_state) {     parent::validateForm($form, $form_state);      $migration_id = $form_state->getValue('migrations');     $definition = $this->pluginManagerMigration->getDefinition($migration_id);     $migrationInstance = $this->pluginManagerMigration->createStubMigration($definition);     $extension = $this->getFileExtensionSupported($migrationInstance);      $validators = ['file_validate_extensions' => [$extension]];     // Check to see if a specific file temp directory is configured. If not,     // default the value to FALSE, which will instruct file_save_upload() to     // use Drupal's temporary files scheme.     $file_destination = $this->config->get('file_temp_directory');     if (is_null($file_destination)) {       $file_destination = FALSE;     }     $file = file_save_upload('source_file', $validators, $file_destination, 0, FileSystemInterface::EXISTS_REPLACE);      $nid = $form_state->getValue('verfahren');     $uid = $form_state->getValue('verwalter');      if (isset($file)) {       // File upload was attempted.       if ($file) {         $form_state->setValue('file_path', $file->getFileUri());       }           // File upload failed.       else {           $form_state->setErrorByName('source_file', $this->t('The file could not be uploaded.'));         }       }        else {         $form_state->setErrorByName('source_file', $this->t('You have to upload a source file.'));       }     }    /**    * {@inheritdoc}    */   public function submitForm(array &$form, FormStateInterface $form_state) {      $nid = $form_state->getValue('verfahren');     $uid = $form_state->getValue('verwalter');        $verwalter2 = [         'plugin' => 'default_value',         'default_value' => $uid,     ];       $migration_id = $form_state->getValue('migrations');     /** @var DrupalmigratePluginMigration $migration */      $migration = $this->pluginManagerMigration->createInstance($migration_id);         $migration->setProcessOfProperty('field_akten_verwalter', $verwalter2);             // Reset status.     $status = $migration->getStatus();     if ($status !== MigrationInterface::STATUS_IDLE) {       $migration->setStatus(MigrationInterface::STATUS_IDLE);       $this->messenger()         ->addWarning($this->t('Migration @id reset to Idle', ['@id' => $migration_id]));     }      $options = [       'file_path' => $form_state->getValue('file_path'),     ];     // Force updates or not.     if ($form_state->getValue('update_existing_records')) {       $options['update'] = TRUE;     }     $plugin_defination = $migration->getPluginDefinition();     $executable = new MigrateBatchExecutable($migration, new StubMigrationMessage(), $options);      $executable->batchImport();    }    /**    * The allowed file extension for the migration.    *    * @param DrupalmigratePluginMigration $migrationInstance    *   The migration instance.    *    * @return string    *   The file extension.    */   public function getFileExtensionSupported(Migration $migrationInstance) {     $extension = 'csv';     if ($migrationInstance->getSourcePlugin() instanceof CSV) {       $extension = 'csv';     }     elseif ($migrationInstance->getSourcePlugin() instanceof Json) {       $extension = 'json';     }     elseif ($migrationInstance->getSourcePlugin() instanceof Xml) {       $extension = 'xml';     }  return $extension; } } 

Code from my .yml file:

id: confidence_migrate migration_tags:   - CSV label: 'Daten der Akten importieren' source:   plugin: csv   ids: [Lfd. Nr.]   delimiter: ";"   header_offset: 0 process:   title: Bezeichnung   field_akten_halle: Halle   field_akten_gang: Gang   field_akten_jahrgang: Jahrgang   field_akten_objekttyp: Objekttyp   field_akten_regalboden: Regalboden   field_akten_regalteil: Regalteil   field_akten_beschreibung: Inhalt   field_akten_lagerung_bis:     plugin: format_date     from_format: d.m.Y     to_format: Y-m-d     source: Lagerung   field_akten_verwalter:     plugin: default_value     default_value: {  }   field_akten_auswahl_verfahren:     plugin: default_value     default_value: {  }    destination:   plugin: 'entity:node'   default_bundle: akten migration_dependencies:   required: {  }   optional: {  } 

My CSV file looks like this:

Lfd. Nr.;Halle;Gang;Regalteil;Regalboden;Objekttyp;Jahrgang;Bezeichnung;Inhalt;Lagerung bis (Datum);nid;uid 

I would appreciate any help thanks.