class AjaxFormBlock

class AjaxFormBlock extends BlockBase implements FormInterface, ContainerFactoryPluginInterface {
    
    /**
     * The form builder.
     *
     * @var \Drupal\Core\Form\FormBuilderInterface
     */
    protected $formBuilder;
    
    /**
     * The messenger.
     *
     * @var \Drupal\Core\Messenger\MessengerInterface
     */
    protected $messenger;
    
    /**
     * Constructs a new AjaxFormBlock.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin ID for the plugin instance.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
     *   The form builder.
     * @param \Drupal\Core\Messenger\MessengerInterface $messenger
     *   The messenger.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, FormBuilderInterface $form_builder, MessengerInterface $messenger) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->formBuilder = $form_builder;
        $this->messenger = $messenger;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('form_builder'), $container->get('messenger'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        return $this->formBuilder
            ->getForm($this);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_forms_test_block';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['test1'] = [
            '#type' => 'select',
            '#title' => $this->t('Test 1'),
            '#required' => TRUE,
            '#options' => [
                'option1' => $this->t('Option 1'),
                'option2' => $this->t('Option 2'),
            ],
            '#ajax' => [
                'callback' => '::updateOptions',
                'wrapper' => 'edit-test1-wrapper',
            ],
            '#prefix' => '<div id="edit-test1-wrapper">',
            '#suffix' => '</div>',
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * Updates the options of a select list.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return array
     *   The updated form element.
     */
    public function updateOptions(array $form, FormStateInterface $form_state) {
        $form['test1']['#options']['option1'] = $this->t('Option 1!!!');
        $form['test1']['#options'] += [
            'option3' => $this->t('Option 3'),
            'option4' => $this->t('Option 4'),
        ];
        return $form['test1'];
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger
            ->addStatus('Submission successful.');
    }

}