code: public function onKernelRequest(GetResponseEvent $event): void
      {
          if (!$this->scopeMatcher->isBackendMasterRequest($event)) {
              return;
          }
  
          $request = $event->getRequest();
  
          if (null === $this->token) {
              $this->token = $this->tokenManager->refreshToken('contao_referer_id');
          }
  
          $request->attributes->set('_contao_referer_id', $this->token->getValue());
      }
docs: Adds the referer ID to the request.