I am trying to export each user after registration from my application to Zoho contacts, but I find no way to generate the access token without front-end user interaction (confirmation &redirection). I tried to use the self-client but I found it very limited! 
The generated code can last for maximum 10 minutes and you can only have 20 refresh tokens for each user then you will have to generate manually a new code!! 
Is there a way to generate an OAuth access token by using the API only? Or by generating permeant access tokens or unlimited refresh tokens instead!