I have a form which provides select2 when the select1 is changed. However I would like select2 to be a multi-select. I can’t seem to get this to work. It populates a single select input just fine, but not the multi-select. Any tips would be appreciated.

public function buildForm(array $form, FormStateInterface $form_state, $options = NULL) {   $form['expeditions'] = [     '#type' => 'select',     '#title' => $this->t('Choose Project'),     '#empty_option' => $this->t('Select a project'),     '#default_value' => '',     '#options' => $options,     '#ajax' => [       'callback' => '::getDeployments',       'wrapper' => 'deployment-wrapper',     ],   ];    // Disable caching on this form.   $form_state->setCached(FALSE);    $form['deployment_wrapper'] = [     '#type' => 'container',     '#attributes' => ['id' => 'deployment-wrapper'],   ];    return parent::buildForm($form, $form_state); }  public function getDeployments(array &$form, FormStateInterface $form_state) {   $expedition_id = $form_state->getValue('expeditions');   if ($expedition_id != "") {     $arms_config = Drupal::config('arms.settings');     $rest_root = $arms_config->get("arms_rest_uri");      $client = Drupal::service('http_client');     try {       $result = $client->get(         $rest_root . 'arms/projects/' . $expedition_id,         ['Accept' => 'application/json']       );       $expedition = json_decode($result->getBody());     }     catch (RequestException $e) {       watchdog_exception('arms', $e);       drupal_set_message('Error fetching project.', 'error');     }   }    $options = [];    foreach ($expedition->{'deployments'} as $deployment_id) {     array_push($options, $deployment_id);   }    $form['deployment_wrapper']['deployments'] = [     '#type' => 'select',     '#title' => $this->t('Choose Deployment(s)'),     '#options' => $options,     '#multiple' => TRUE,   ];    return $form['deployment_wrapper']; }