Enable a custom field only for staff (privileged) users

1. Pick the Admin > Global > Custom Fields > Tickets menu option
2. Click your custom field (Location)
3. Click the Group Rights tab
4. Click Privileged
5. Click View Custom Fields on the General rights tab to give read access
6. Click both rights on the Rights for Staff tab if you'd like them to be able to edit your custom field
7. Click Save Changes