Garner is straight up right but if you are trying to get Fullscreen in the editor while previewing, hit ‘F11’ I believe it is. (it’s in editor keybinds)
If you are trying to get the Editor to do this automatically you have to set it in a config file and save it in a certain place to get it to work. (search answer hub for this solution)
If you want it to run Fullscreen in ‘Play’->‘StandAlone Game’ or in a finished or packaged game, you can do this:
(normally would use a flip/flop instead of a branch but it works more consistently this way)
(this is an UMG Menu example, the event is a button that is clicked from UMG)
(you can do this almost anywhere by either setting an action event to a key in project settings or just right click & hit the key you want for a key event node in the BP)
(console command node works for screen resolutions as well just have to be careful with it, unless you are polling the users settings etc. in an actual game & I can’t remember exactly where it is but you can do the same w/ screen resolution just by adding a w or and f but vague on that exactly)(search for this info it’s out there)
(still hoping they expose & setup a cleaner easier way to handle user settings like this but for now this is it, as far as I know)