Create related record in non-relational FP

Using FP5, but since this is probably a pretty basic question, and the answer will probably be the same for all non-relational versions, here goes:

File one is Members, file two is Classes. File relationship is on MemberID in both files.

I want a button in the Edit Members layout to open the Class file, create a new record, and insert the Member ID into the Member ID field in the Classes file.

What would be the step in the script to insert that value?