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.
Click the lock icon to unlock it, then enter an administrator name and password.
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.