Go into Video settings and manually set a resolution of 1920x1080.
Then in your desktop/game capture, right click on it and go to Filters. Apply a Scale/Aspect Ratio filter, and in the Resolution drop down select 16:9 and check the box that says "Undistort center of image when scaling from ultrawide". This will make it so that when you scale the image to a different aspect ratio, instead of looking squished, it will look pretty normal in the center and instead distort the edges.