Prometheus datasource browser

Try to setup grafana to use an external prometheus source.
Curl show the result page when i try to connect from the same system.
After adding source and select Browser Grafana is doing 0 network traffic to this source.

Using version 7.5.10

From doku here:

Cant see my mistake.

Change the Access from Browser to Server(default)

Did not change anything but traffic is generated.

Change the GET to POST

i’ve tried the minor changes already