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/**
   * {@inheritdoc}
   */
 public function buildForm(array $form, FormStateInterface $form_state) {
 
   $form['leasing_options'] = [
      '#type' => 'details',
      '#title' => $this->t('Mileage / Duration'),
      '#open' => TRUE,
      '#ajax' => [
        'callback' => '::ajaxModalCallback',
        'event' => 'click',
        'progress' => [
          'type' => 'throbber',
        ],
      ],
    ];
 }
  
  
  /**
   * AJAX callback for the Mileage/Duration table modal.
   *
   * @param array $form
   *   The form array.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state object.
   *
   * @return \Drupal\Core\Ajax\AjaxResponse
   *   The AJAX response.
   */
  public function ajaxModalCallback(array &$form, FormStateInterface $form_state) {
    $response = new AjaxResponse();
    $settings = array(
      'dialogClass' => 'adopt-modal',
      'width' => '700',
    );
    // see all commands here: Drupal\Core\Ajax 
    $response->addCommand(new OpenModalDialogCommand($title, $content, $settings));

    return $response;
  }