Well I cant start a run when im tabbed out of it.

Example: I don't do speedruns for on here but I speedrun among us so I have livesplit timer, I am tabbed into among us and not it but it is on the same monitor, and I press my mouse button to start a run but it does not start or same with ending, I have to tab into it.

Is there a setting that can fix this? Also, what's a good CPU for a low price? I might buy one