You can have custom lockscreen messages show up on Mac OS X 10.7 or later. There are various third party apps which allow you to add custom lock screen messages in your Mac OS X. However you can do so without downloading any app.
Mac custom messages have enough space for informative lock screen messages without any word limit. The message itself can be anything: a message to return your stolen Mac to you or any information for guest users.
Here is How You Can Add a Customized Lock screen Message in OS X:
Method 1: Use “System Preferences”
Step #1. Select “System Preferences” from the Apple Menu.
Step #2. Choose “Security and Privacy” option.
Step #3. You might have to do this: Click on the “Lock” icon below the General tab window. You will have to add your username and password to be able to make changes.
Step #4. Check the option, “Show a message when the screen is locked.” Now, click on “Set Lock Message.”
Step #5. In the next window, type the message you want to display on your lock screen.
Step #6. Press “OK.”
Method 2: Using “Terminal”
Step #1. Click on “Application” and then on “Utilities.”
Step #2. Now, select “Terminal.”
Step #3. In the following window, type the command :
sudo write /Library/Preferences/com.apple.loginwindow LoginwindowText "Custom Text Here" (Note that, you have to replace the “Custom Text Here” with the message you want to display on your login screen.)
If you wish to remove or change the text, here is the command for it
sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText ""