$ajaxResponse = new \Drupal\Core\Ajax\AjaxResponse();
    $my_form = \Drupal::formBuilder()->getForm('Drupal\my_module\Form\DefaultSettingsForm');
    $render = \Drupal::service('renderer')->render($my_form);
    $modalDialog = new \Drupal\Core\Ajax\OpenModalDialogCommand('My Title', $render);
    $ajaxResponse->addCommand($modalDialog);
    return $ajaxResponse;