First of all remember that you cannot test the x-family command in "Preview Application".  You either need to save as different application or merge with the existing application to test it.

Since it is the same application, the application name will be @fn,  say you are trying to look up Name from database A while working with database B.  the form name is Main in A database

Database A:

id:                 Name:

******************
Database  B:

id:                NewName:

syntax will be:

NewName = @xlookup (@fn, id, "Main!id", "Name")