You will need an administrator account to create a lock screen message on your Mac, but it's super easy to do.
- Click on the "" icon in the menu bar.
- Select System Preferences.
- Choose Security & Privacy.
- Select the "General" tab.
- Click on the lock icon at the bottom of the window.
- Enter your admin name and password.
- Choose "Set Lock Message..."
- Enter your desired message.
If you misplace or Mac or have it stolen, this puts your contact information front and center as soon as your Mac is opened up, so whoever has it can see who it belongs to even before you activate Lost Mode using iCloud. It's also potentially useful in an emergency if someone needs to know who you are and how to get in contact with a loved one.
I'm personally always paranoid about losing my MacBook or having it taken at some location like an airport security lane, so this is a nice way to have instantly identifiable info readily available.