TestForm.php

<?php

namespace Drupal\block_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Form that performs base block form test.
 *
 * @internal
 */
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 $form_state) {
    if (!str_contains($form_state
      ->getValue('email'), '.com')) {
      $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'],
    ]));
  }

}