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.