Custom expression in VBS not available/ possible?

Summary:

While creating a defaulting rule in redwood DOR page, We need to default one of the field values as 'Legal first name + Legal last name'. We tried using the oracle delivered expression 'Full Name' but that defaults the field to 'Preferred First Name + Preferred Last Name'. We tried creating a custom expression but could find a way to reference the PER_PERSON_NAMES_F table to fetch the FIRST_NAME & LAST_NAME columns. Can this be achieved. If yes, how?

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