Update help- Custom record association with customer parent

This post is for the most part organized into three questions/categories, and in summary, my questions are about the database structure behind NetSuite, and how to associate a custom record with a customer record so it doesn't create duplicates.

Details of my questions:
How would one get NetSuite to associate the material request records and customer records?-
I'm setting up a custom record that takes data from an online form and pushes it into the record. The problem is that in most cases, the customer already exists (with their own unique identifier established). One would think that Netsuite would automatically search new custom records and check with existing customer records(since it is the parent), and push out the custom record into the customer marketing subtab for materials requests(since this is the name of the chosen subtab under the marketing subtab that the fields are pushed out to - although really, the relational database behind it all must