views: pass service field to fc login post registration (fixes #21908)

src/authentic2_auth_fc/views.py

@@ -459,6 +459,8 @@ class RegistrationView(PopupViewMixin, LoggerMixin, View):
         params = {
             REDIRECT_FIELD_NAME: redirect_to,
         }
+        if constants.SERVICE_FIELD_NAME in request.GET:
+            params[constants.SERVICE_FIELD_NAME] = request.GET[constants.SERVICE_FIELD_NAME]
         if self.get_in_popup():
             params['popup'] = ''
         redirect_to = a2_utils.make_url('fc-login-or-link', params=params)