Correct aspect ratio/resolution for LG 35WN65C when playing games on Xbox

Unfortunately this isn't possible. Your display is an Ultra-Wide (3440x1440) and as yet Xbox consoles don't support Ultra-Wide 21:9 aspect resolutions.

They only support the more common 16:9 aspect resolutions such as 720P (1280x720), 1080P (1920x1080), 1440P (2560x1440) and 4K (3840x2160).

These monitors are really designed for PC usage rather than game consoles. Unfortunately this means you're either going to be stuck with either a stretched image or one with bars at top and bottom of the display.

Check your monitor’s OSD settings. There should be an option for aspect ratio, but the

LG manual states that if you’re already at 2160p (4K) resolution, there shouldn’t be any stretching in the first place.

Also verify in the Xbox display settings that you’re at 4K resolution.

On the TV remote control, find the SCREEN or Aspect or Aspect Ratio button. And set the correct screen resolution.

Mostly, this button is marked with a screen image and four arrows to the sides.

I am unsure what you mean by "widescreen" as all retail games--that I'm aware of--since the early 2000s have been in a 16:9 resolution because of the wide-spread adoption of televisions in that aspect ratio. 720p and 1080p both natively display at a 16:9
aspect ratio. The only screen resolution that the Xbox 360 supports that is not at that aspect ratio is 480p which is in a 4:3 ratio.

21:9 aspect ratio

Hi,

There is your issue. Xbox One only supports 1440P on 16:9 aspect ratios (2560x1440).

Xbox supports your vertical resolution (1440), but not your display's horizontal resolution of 3440.

I have no idea if MS will support 21:9 aspect ratio's on Xbox One in the future, although I believe Series X will.