And this is true for your PM's as well. So for me the easiest thing is just to go to my messages and click from there. Unless it's a zap and you don't have a message, and then you have to find the person in the forum.Under their profile picture anywhere they posted is an "Add Feedback" link.
Note that it will use the thread as the one you are leaving feedback about. So if you are clicking it from somewhere other than the relevant thread, clear that out on the input screen.