Issues with Zoho Sign API

Hi,

Objectives

I have a custom solution (based on a MEAN - MongoDB, Express, Angular, Node - stack) and am looking to integrate it with our Zoho CRM (and other modules).

One of the aspect I'm particularly interested in is Zoho Sign as part of our validation Workflow for both: Purchase Orders and Timesheet.

I would like to programmatically issue a Zoho Sign request from my custom solution and act upon completion of the Zoho Sign signing process.

Progress so far

I have looked at this 2 very interesting documents:

https://docs.zoho.com/writer/open/4jpyv16e652fe678c4de5abaa950f8f3edff9

https://docs.zoho.com/writer/open/3srml39affc8894854871b8514b5356f85895

and have been able to get the token & upload a document.

Questions

Grant token

I am able to start the process by issueing a GET request from the browser

https://accounts.zoho.com/oauth/v2/auth?scope=ZohoSign.documents.ALL&client_id=1000.581V5TZRBxxxxxxxxx&response_type=code&access_type=offline&redirect_uri=http://localhost:3000

and having a local web server takes care of the end of the process

When I try to do the same from NodeJS (or Postman), it failed as I think a web page is returned with Javascript in it to redirect...

Am I missing something?
Create Signature Request

I successfully managed to have my document uploaded but then got an Error 4002 stating that the data in invalid (without further information).

Many thanks in advance for your help.

Best regards,

Guillaume