I tried to use a SurfaceGui TextBox, but in-game, it didn't let me change the text. Tips?
Unfortunate to say, but this feature is ROBLOX Locked. Surface Gui Text Boxes can be used to host inappropriate content from other players, thus bringing moderation of the place into question. This is more of a time save for moderators, and a stress free for game developers.
ROBLOX Locked - The locking of a particular function, method, or event for safety and security of a developer's place or game. These locks can not be lifted.
Edit; you could have a player hit a button and be prompted with a Screen Gui to modify the message on the Surface Gui.