Display a message in the Mac login window
You can add a short message to the login window that is displayed when your Mac is locked. The message can be anything appropriate to users attempting to log in — for example, to provide information for guest users. The message can also act as a virtual engraving, to provide contact information for a misplaced computer.
On your Mac, choose Apple menu > System Preferences, click Security & Privacy, then click General.
If the lock at the bottom left is locked , click it to unlock the preference pane.
Select “Show a message when the screen is locked”, then click Set Lock Message.
Enter the message you want to display in the login window, then click OK.