duplicating the "create warranty" checkbox when creating IB records via APIs.

We have an item that is in a BOM, associated with a warranty item.  When I create a new IB record by hand for this item, there is a checkbox called 'Create warranty" that is by default checked.  When I save this new IB record, a new warranty contract is immediately created.

However, when I call the API (create_item_instance) to create an IB record for this same item, no warranty contract is created.  I don't see any field in the API records to rerplicate that "create warranty" checkbox that is found on the on-line screen. 

Does anyone know how to automatically get the warranty contract created when you create an item instance through the APIs?  Does it create the warranty contract if you create the new instances through the open interface?  I need to write a program to create multiple new IB records (hundreds of 'em), so I need to find an automated way to generate these warranty contracts.