Whether you just want to have fun or if your want to put a message up for users, you can create a customer login. An individual can do something creative, or if your a business maybe you want to put up an “if found” message or even a “this property belong to” message to help ensure your Mac is returned to where it belongs.
sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText “In case of loss, call 555-555-5555.”
In this example you can put any message in between the quotation marks and it’s really that simple. Just follow the syntax above and you have your own custom login message.