Usability can be tested in two ways;
Within explorative tests, usability suggestions are usually welcome.
Here, you can report improvements of the software or a function, or report unexpected behavior in short sentences with screenshots. For Usability suggestions, you will get a smaller bonus compared to functional bugs.
There are also special Usability tests where the testers have to record “thinking out loud” videos or use other methods to submit their feedback. The payout depends on the project and estimated effort for you.