How to create an agreement with FormFields placed - what else am I missing?

Been trying to create an agreement with form fields adding using the REST V6 API. We can't use a template as the PDF is generated in code and can have unique content and branding.

This is what I have so far:

1. Create transient document from PDF
2. Create an agreement as AUTHORING, pointing to transient, with a single signer
3. Get the agreement data back, so I can get the the Assignee ID for the participant
4. Use PUT to add form fields, setting the Assignee value as above
5. Send agrement by PUT to change status to IN_PROCESS

When I do this last step, I get a 401-forbidden, OPERATION_NOT_ALLOWED error. HOWEVER the agreement is sent out???!!

Another question: can I put two signature blocks for separate parts?? We have a direct debit agreement on the same page which needs its own distinct signature. This is possible on a "certain other competitor platform".