code: public function start(Request $request, AuthenticationException $authException = null)
      {
          if ($request->query->count() < 1) {
              return $this->httpUtils->createRedirectResponse($request, 'contao_backend_login');
          }
  
          $url = $this->router->generate(
              'contao_backend_login',
              ['referer' => base64_encode($request->getQueryString())],
              UrlGeneratorInterface::ABSOLUTE_URL
          );
  
          return $this->httpUtils->createRedirectResponse($request, $url);
      }
docs: {@inheritdoc}