How to run game with 16:9 aspect ration for 21:9 monitor with black bars?

Hello everyone.

My monitor 21:9 (2560x1080) and want to play with 16:9 aspect ratio (1920x1080) with black bars, left and right (because with a resolution 2560x1080 - graphics card loaded at 99% and overheats a lot, but when i do 1920x1080, the load drops to 70% and everything returns to normal).
I tried to set fullscreen mode, then the black bars work, but i want to make the game quickly collapse after alt+tab, so need windowed mode. But if i set a windowed fullscreen, then game is made to fullscreen + stretched.

So the question is: how to make a game with a 16:9 aspect ratio with black bars? I have been playing in windowed mode for several years now, but I always see the desktop to the left and right of the game, which is very inconvenient.

ask support

This what would be great for me too, let us know if there's a solution!