During the game enter system options — there’s checkbox labelled “Fullscreen”. Game should toggle fullscren when you check it. Or you can just press F4 during the game. (For some reason F4 doesn’t seem to work in pregame… you can report that)

If for some reason you really need to switch fullscreen setting without entering game, edit settings.json file in config subfolder. In the “video” section add the entry
“fullscreen” : true

Tow helped me with it.

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