Question How do I set a custom windowed resolution?

[CalDaMan]

I run Mortal Online 2 in windowed mode at 1366x768 and would like to make the window wider maybe something like 21:9. My monitor is 16:9 4K so I don't have any resolutions in the game that are wider than 16:9. The game is launched through steam if that's relevant.

ANY help will be greatly appreciated,
Thanks