Custom BuddyPress Registration Form That Updates Specific Field

I'm using a combination of BuddyPress and WPMU Dev's plugin 'Membership Premium.'

What I want to accomplish is:

• Use a custom BuddyPress registration form (I've followed appropriate template hierarchy to have a 'child form,' which has been tested and works)


• I want to create a row in the table ua_m_membership_relationships


• Ideally this row would be populated by hidden fields

My issue is: what is the correct script to populate a row for a specific table for a given user id upon user registration?

Thanks!