Adobe Acrobat Sign module - Make an API Call with multipart/form-data

I have been having trouble using the Create Agreement part of this module where I cannot get the document in the Agreement to work : https://community.make.com/t/adobe-acrobat-sign-create-agreement/67114

To get around this problem I thought I could use the Make an API Call section of the same module and do it manually with a bit more work.

According to the Adobe Sign API documentation, you need to do this in two parts:

  1. Upload the file to a temporary area on their cloud (/transientDocuments)
  2. Create the Agreement referencing the uploaded file’s ID (/agreements)

The issue I am struggling with is point 1 - using the /transientDocuments POST
https://secure.na1.adobesign.com/public/docs/restapi/v6

This API call needs multipart/form-data and the Make and API call area of the Adobe Acrobat Sign module does not mention how I can do this.