You don't have to create a user define field.
Go to Mandatory Keys dialog and add your field as mandatory with SQL Statement like this:
select 8 (for integer datatype)
select '8' (for varchar)
If you have link between for example NT user and the privileges table in your DB, you can also use a dynamic statement like this:
select privileges from MyPrivileges where NTUser='{WINDOWS_NT_USER}'
(see Filter Rows Help topic for possible params)
GeniusConnect will run your SQL Statement
before insert, and the result value (8) will be inserted in your DB in field "privilegios"