As far as I can tell, Sonarr cannot communicate with download clients behind a reverse proxy if they’re not at the root url.
So this feature request is to add a field to the download client configuration dialog that lets you input a path.
For example, if you use nginx to put nzbget at https://foo.bar.baz:34562/nzbget, Sonarr cannot communicate with nzbget because there is no way to tell Sonarr to add /nzbget to the url.