By default, Steam uses this command to launch the game:
/home/perchun/.var/app/com.valvesoftware.Steam/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=1874880 -- /home/perchun/.var/app/com.valvesoftware.Steam/.local/share/Steam/ubuntu12_32/steam-launch-wrapper -- '/home/perchun/.var/app/com.valvesoftware.Steam/.local/share/Steam/steamapps/common/SteamLinuxRuntime_sniper'/_v2-entry-point --verb=waitforexitandrun -- '/home/perchun/.var/app/com.valvesoftware.Steam/.local/share/Steam/steamapps/common/Proton - Experimental'/proton waitforexitandrun '/home/perchun/.var/app/com.valvesoftware.Steam/.local/share/Steam/steamapps/common/Arma Reforger/ArmaReforger_BE.exe' -exe ArmaReforgerSteam.exe
So I set this as custom command (similar to https://www.protondb.com/app/1874880#pjDWqulZwY)
echo "%command%" | sed 's/ArmaReforgerSteam.exe/ArmaReforger_BE.exe/' | sh
And now game shows as launched, hangs out for ~10 seconds and exits. There is no window on the screen, so this should be a wrong executable to launch.