Q. Setting a Lock Screen Message on Your Mac
Apple has provided a way for you to leave a message on your Mac's lock screen. This is a good way to provide contact information if your Mac is lost or stolen and falls into the hands of a good Samaritan.
The process for setting a lock message may vary as Apple releases new versions of macOS. To see the correct instructions for the version of macOS you're running, read Apple instructions here: "Display a message in the Mac login window" in the macOS User Guide
I find it useful to include something along the lines of "Property of Carthage College. If lost, contact [NAME], [USERNAME]@carthage.edu, 262.551.[EXTENSION]". For me, that's "Property of Carthage College. If lost, contact Richard LaHue, RLAHUE@carthage.edu, 262.551.5793"
To check the version of macOS you're running, go to Apple menu () → About This Mac.