class DummyAjaxWidget

class DummyAjaxWidget extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $element['select_widget'] = [
            '#type' => 'select',
            '#title' => $this->t('Dummy select'),
            '#options' => [
                'pow' => 'Pow!',
                'bam' => 'Bam!',
            ],
            '#required' => TRUE,
            '#ajax' => [
                'callback' => static::class . '::dummyAjaxCallback',
                'effect' => 'fade',
            ],
        ];
        return $element;
    }
    
    /**
     * Ajax callback for Dummy AJAX test.
     *
     * @param array $form
     *   The build form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The form state.
     *
     * @return \Drupal\Core\Ajax\AjaxResponse
     *   Ajax response.
     */
    public static function dummyAjaxCallback(array &$form, FormStateInterface $form_state) {
        return new AjaxResponse();
    }

}