Create a one record table that will only be accessed by the person that has the ability to edit the "message". In that table create a field or data type text that will be the repository for the message. This field should NOT have global storage.
Then create a message field that is of type calculation set to global storage that gets the value in message.
Yes, you want the single-record preference table method and a standard field. NOT a global field.
A global field is single-user single-session.
With some difficulty, a global can be populated by automation methods (scripts) so that it holds a common, updated value for all users and sessions.
That won't work. It will not update.