core-bundle/src/Security/Authentication/AuthenticationEntryPoint.php
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);
    }
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);
    }
{@inheritdoc}