Liked or Thumbs UP button.
How you find and activate the 'thumbs up' button?
Dashboard/Forums/Settings/Forum Access - check "can like" for each access mask you want it enabled for.
More info here:
The location of the like button can differ depending on the layouts you're using.
More info for wpForo layouts here: https://wpforo.com/docs/root/categories-and-forums/forum-layouts/
Also please note the users cannot like their own posts.