Are there any shortcuts for the serverMessage command?

Yes, you can also use ";sm" or ";smessage".

What rank do I need to use the serverMessage command?

You need the Admin rank or higher in HD Admin to use this command.

Type ;serverMessage [Message] in the chat while playing a game with HD Admin installed.

No, this command only works in games that have the HD Admin plugin installed by the game developer.

HD Admin Admin

The serverMessage command sends the specified message as a game message to everyone on the game.

This command sends the specified message as a game message to everyone on the game.

;serverMessage [Message]

Example: ;serverMessage hello server

This command sends the specified message as a game message to everyone on the game. The message is visible only to players currently in the game.