As far as I can tell envelope forard isn't documented anywhere in the Docusign API and it took quite a bit of digging to figure it out. Here's how it works:
Forwarding creates a copy of a completed envelope which you can send to new recipients as is or add new documents and fields as needed. For a forwarded envelope, you cannot remove any information or signatures on the completed documents, but you can add new recipients, documents, and recipient fields.
https://support.docusign.com/en/guides/ndse-user-guide-forward
POST to /envelope as you would if you were creating a fresh new evelope.
The trick is that you need to add completed_documents_only=true to the url query string. Without this you will
just get a copy of the envelope with all the signers and field information blanked out.
[POST] /accounts/{ACCT_ID}/envelopes?completed_documents_only=true
// body { EnvelopeId: {ID of the envelope you want to forward} ... ... Standard create envelope params }