ElasticSearch Auth without user in datasource


we have an ES 7 cluster with enabled (Basic) Auth. If I create an ES User and configure it in the Grafana datasource with Access Type “Server”, everything works fine.
Now I want to let the user enter the ES user and password. I tried to let username and password empty for the datasource. Then the browser opened the basic auth window, but it seems not to work with the same credentials as in the datasource before. I just get an 401 error. Maybe the credentials were lost through the proxy?

Is there a way to let the user enter the User/PW and it will be used for the datasource auth? Maybe switch from Proxy, to Browser Auth and enable CORS? But this would only be a last resort.

Thanks and regards