How to configure my Streamlab OBS to capture my CSGO full screen?

I have a small problem about my Streamlabs OBS. It can't capture full screen on my CSGO. As you can see below this image the yellow circle, there's some more space on it when I'm streaming.

So the question is how can I fill up those space? Is there any setting that can allow 4:3 display into 16:9? Maybe?

I actually have thought is it because my in-game resolution have interrupt but I'm not sure. This is my in-game resolution. If you wonder why my in-game aspect ratio is 4:3, that's my preference.

If isn't my in-game resolution affect the game, is there anyway to setup?

You can change this within Streamlabs.

Right-click the screen itself and go to 'Transform'. From there, stretch it to fit the screen.

That should fix it.