code: public function loginAction(): Response
      {
          $this->get('contao.framework')->initialize();
  
          if (!isset($GLOBALS['TL_PTY']['error_401']) || !class_exists($GLOBALS['TL_PTY']['error_401'])) {
              throw new UnauthorizedHttpException('', 'Not authorized');
          }
  
          /** @var PageError401 $pageHandler */
          $pageHandler = new $GLOBALS['TL_PTY']['error_401']();
  
          try {
              return $pageHandler->getResponse();
          } catch (ResponseException $e) {
              return $e->getResponse();
          } catch (InsufficientAuthenticationException $e) {
              throw new UnauthorizedHttpException('', $e->getMessage());
          }
      }
docs: Symfony will authenticate the user automatically by calling this route.
  
  @return RedirectResponse|Response
  
  @Route("/_contao/login", name="contao_frontend_login")