You can use the UUID - via script
- when checking the field (script trigger):
set Field (MyField; GetUUID) // or similar, don't have an english system here
You can make a new "Funders" table and make the checkbox a button (or related field) that creates a record in the new table. The ID would be an auto-enter serial in the Funders table.
You can use an auto-enter calculation something like this:
Case ( not IsEmpty ( Funder ) ; Get ( UUID ))
Edit: Sorry, missed the "serial number" part. If you want a serial number, you can use a summary field that uses List of the serial, then change the auto-enter calc to read:
Case ( not IsEmpty ( Funder ) ; Max ( cList ) + 1 )
where cList is the summary field.