You can set a persistent message that is displayed in the header at the top of your chat room window.  You can do this when you edit your chat room, but at times you may want quick access to change it 'on the fly'.  Room administrators can change the 'static message' at any time from the admin console window.

Open the admin console by clicking on the menu icon at the top right of the chat room (it looks like 3 horizontal lines).  Now select 'show admin console' from the menu that appears.

When the admin console opens the 'general' tab in the header will be selected by default, and the first thing that you see will be the 'set room topic / message' entry box.  Erase any text that is already there and enter your own message.

Be sure to click on the 'apply' button when you are finished.

Your message will immediately change for all to see.