class TestForm

class TestForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'block_test_form_test';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['email'] = [
      '#type' => 'email',
      '#title' => $this->t('Your .com email address.'),
    ];
    $form['show'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    if (strpos($form_state->getValue('email'), '.com') === FALSE) {
      $form_state->setErrorByName('email', $this->t('This is not a .com email address.'));
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->messenger()
      ->addStatus($this->t('Your email address is @email', [
      '@email' => $form['email']['#value'],
    ]));
  }

}