How to record a 2560x1440 16:9 game with a 3440x1440 21:9 monitor?

[keiqwx]

Hi. I'm using a ultrawide 21:9 1440p monitor and I want to record the game in 16:9 resolution. But in the output settings, I can only set 1920x1080. Why can't I use 1440p and how can I get it?

Spoiler

 

[Fenrir]

That setting is a combobox. You can select something from the list, or just edit it like a text field and type anything you want.