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'],
        ]));
    }

}