Answer:
After you thank them or rate them (I'm honestly not sure which, but why not do both if you're going to do one anyways), a yellow button will appear (I think next to the 5 stars) that says 'mark as Brainliest'. If you click that button, the answerer will get a notification saying that you marked them as Brainliest.
P.S.: Only the person who asked the question can mark an answer as Brainliest, and only one person can be marked as Brainliest.
