You have three ways to set the login window message: using OS X Server, with Terminal, or by using the Security & Privacy system preference pane.
Login Message with macOS Server
With macOS Server, you can create a single message and then send it to all of the connected computers. Use the Workgroup Manager tool to set the login message. Once set, the message is propagated to all of the Macs that connect to the server.
How to Set the Login Message for Individual Macs Using Terminal
You don’t need Server to customize the login message. Two other methods are available. The first uses the Terminal application.
How to Reset the Login Window Message Back to Its Default
To remove the login message text and revert back to the default value of no message being displayed, perform the following steps:
How to Set the Login Message for Individual Macs Using the Security & Privacy Preference Pane
Using a system preference pane may be the easiest method for setting up a login message. The advantage is that you don’t need to work with Terminal and text commands.
How to Reset the Login Message From the Security & Privacy Preference Pane
If you no longer wish to have a login message be displayed, return to the General tab of your Mac’s Security & Privacy preferences and remove the check from the box next to “Show a message when the screen is locked.”
