Inject Mail Manager to Form

use Drupal\Core\Mail\MailManagerInterface;

...

 /**
  * The mail manager.
  *
  * @var \Drupal\Core\Mail\MailManagerInterface
  */
  protected $mailManager;

 /**
  * Constructs a new EmailExampleGetFormPage.
  *
  * @param \Drupal\Core\Mail\MailManagerInterface $mail_manager
  *   The mail manager.
  */
public function __construct(MailManagerInterface $mail_manager) {
  $this->mailManager = $mail_manager;
}

 /**
  * {@inheritdoc}
  */
public static function create(ContainerInterface $container) {
  return new static($container->get('plugin.manager.mail'));
}