The easiest way to do it is type
/Hot Assist /assist Fakktoo (or whatever name you want to assist)
The other way is on your Actions window (where you have Sit, Camp, Follow, Disband) click on the "Socials" tab which is the furthest to the right, right click on any of the default ones (Wave, Duel, Consider ect) or go to pages 2-10 which will all be empty, erase everything, then type /Assist <Player name> on one of the lines and save it.