You searched for CHANNEL NOTICE
1. channel notice
Syntax: @ notice <channel name> ("none")
If you are a primary owner of a channel, you can write the channel's notice
message. This message will be displayed to any player who joins the
channel. The message should be short (no longer than a small paragraph).
It can display anything you wish new channel joiners to know about your
channel. You can clear your channel's notice in the editor or with the
above syntax.