To make the document IDs easier to fill out in your payload, you can specify a custom document ID when creating your transaction. This way, you don’t have to find out the random ID assigned to the document by OneSpan Sign.
Also, remember that with the OneSpan Sign REST API, you will need to get a session token for the appropriate signer, as mentioned in the guide above, to call the auto sign endpoint for the signer.
Hopefully this solves your issue. Please let us know.