How to update pk2_value in FND_Attached_Document for an receivables invoice attachment

The seeded "Print Receivable Transactions" program is populating pk1_value, pk2_value, pk3_value, pk4_value, and pk5_value in Fnd_attached_documents when invoice print is attached to an invoice.
We are using the following API to attach custom documents to a receivables invoice:
REST API used: POST - /fscmRestApi/resources/11.13.18.05/invoices/{invoicesUniqID}/child/attachments

This API only populates PK1_value  in FND_Attached_Documents, we need to populate PK2_value too.
No API documentation found on how to update pk2_value in FND_Attached_Document for an receivables invoice attachment.
Has anyone done this before, if so, what is the solution ?