In your library, each title has a settings button below it. Open that, then go to the Advanced tab. The Game Arguments textbox is what you're looking for.

Game Settings > Advanced > Game Arguments

This is where you'd enter any args the same way you would on windows. Everything else is wine/linux specific stuff.

(No need for %command% like in steam when running a game through proton)

You can use the little "help" button to view tooltips about each config option in Heroic. This one for example says how the field is for stuff like "-nolauncher" when you click it.