In the tutorial it says"Press J to open up all the chats" and for convinient purposes the Support chat is the top most one. So if you press J and type, you are basically typing in support chatAlso from time to time you get the notification of to Press J and type In Support chat to ask any questions on top of your screen
What about if '/sup' is used by a player with less than 15 hours of play time, the "show in chatbox" will be auto ticked after they did the command?