I am new to WPForums and I would like to know if the @username comes with a dropdown to select the username. I saw it was suggested in this post: https://wpforo.com/community/how-to-and-troubleshooting-2/mention-user-list-pop-up/. It could be an error on my side as well.
I'm sorry, but there is no such a feature in wpForo. You can find users @nicknames (this is the nickname this is not username) next to their Display Names or Avatars: