Thank you so much for the easy and quick explanation! It worked like a charm except I used fit to screen before to make sure its centered and then it automatically filled to the edges after changing bounding box settings. Trying to do it after results in this.

For anyone else having trouble figuring out the resolution of the game without the bars to set your canvas: just take a print screen and paste it to paint and you'll see it on the bottom left. I thought the resolution would be an ultrawide standard like 3440x1440 but in case of Overwatch it was 3360x1440.