Vendor Contracts BAPI

I'm writing an LSMW using the below object:

Business Object               BUS2014                                        Purchase contract

Method                             CREATE                                         Create

Message Type                  PURCONTRACT_CREATE           BAPI to Create a Purchase

Basic Type                        PURCONTRACT_CREATE01       BAPI to Create a Purchase

My question is:

How do I link the header and item data, there doesn't seem to be a field to show which records match up to which?