what is the serial number

Share your location in Find My on Mac

1. In the Find My app on your Mac, click People.
2. Select Me in the People list, then click the Info button on the map. ... 
3. Turn on Share My Location (if it's not already on).
4. To allow location sharing requests, turn on Allow Friend Requests (if it's not already on).

Share your location in Find My on Mac - Apple Support

If Find My us not already enabled, there is no means for a user to show the location; neither the serial number nor IMEI is sufficient. Find My must already be enabled, and cannot be enabled remotely.