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Select game title on left panel
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Right Button → Properties → Set launch options…
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Enter desired variables, separated by space and ended by
%command%
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You want to create log file to check if everything fine or file a bug, use Environment variable:
PROTON_LOG=1 %command%
Log will be saved at location, where [appid] is your game id (look up steamdb if in doubt)
~/steam-[appid].log