public function getFormId()
  {
    return 'registration_form';
  }

public function buildForm(array $form, FormStateInterface $form_state)
{
 $form['registration_form']['lastname'] = [
        '#type' => 'textfield',
        '#title' => $this->t('lastname'),
        '#required' => FALSE,
        '#default_value' => (isset($profile->lastname)) ? $profile->lastname : '',
        '#prefix'      => '<div class="form-group">',
        '#suffix'      => '</div>',
      ];
}

public function validateForm(array &$form, FormStateInterface $form_state)
  {
/* if (!$form_state->getValue('firstname') || empty($form_state->getValue('firstname'))) {
      $form_state->setErrorByName('firstname', $this->t('firstname is required'));
    }
}

public function submitForm(array &$form, FormStateInterface $form_state)
{
Redirection;
}