I have a similar problem as daz1st, I'm running EndeavourOS as distro.

I have followed the stops throughly on https://wiki.faforever.com/en/Linux-Install

I'm able to run SupCom from steam with no problems, just running SupCom with FAForever that's not working

FAF: ~/.faforever/run "%s" /nomovie
STEAM: WINEDLLOVERRIDES="d3d9=n" PROTON_NO_ESYNC=1 PROTON_DUMP_DEBUG_COMMANDS=1 %command%

From FAForver terminal: https://pastebin.com/TSh3xMRa

Nevermind I found out the problem.

Thought that arch-based put the client in ~/.faforever/
Solution:
cp /tmp/proton_$USER/run /usr/share/java/downlords-faf-client

and then in Command Line Format for Executable in FAF-Client
/usr/share/java/downlords-faf-client/run "%s" /nomovie