I would like to add a submission handler to my form class so that the submitted values are saved. The parent class does not save them but only displays a message. I use Drupal 8! Here is my form class:

<?php  namespace DrupalgreetingForm;  use DrupalCoreFormConfigFormBase; use DrupalCoreFormFormStateInterface;  class GreetingForm extends ConfigFormBase {    /**    * {@inheritdoc}    */   public function getFormId() {     return 'greeting_form';   }  /**    * {@inheritdoc}    */   public function buildForm(array $form, FormStateInterface $form_state) {      $form = parent::buildForm($form, $form_state);      $config = $this->config('greeting.settings');      $form['page_text'] = array(       '#type' => 'textarea',       '#title' => $this->t('Content of the Greeting page'),       '#default_value' => $config->get('page_text'),       '#description' => $this->t('Allows you to define the text of the Greeting page'),     );      return $form;   }    /**    * {@inheritdoc}    */   public function validateForm(array &$form, FormStateInterface $form_state) {    }    /**    * {@inheritdoc}    */   public function submitForm(array &$form, FormStateInterface $form_state) {      $config = $this->config('greeting.settings');      $config->set('page_text', $form_state->getValue('page_text'));          $config->save();      return parent::submitForm($form, $form_state);   }    /**    * {@inheritdoc}    */   protected function getEditableConfigNames() {     return [       'greeting.settings',     ];   }  }  $config = Drupal::config('greeting.settings'); $page_title = $config->get('page_text');```  How do I do it? Thank you, Alexis Grolot