How to identify user, organization in first request to server app

I will be creating a server side app for zendesk support. I have gone through https://developer.zendesk.com/documentation/apps/build-an-app/building-a-server-side-app/bonus-part-secure-the-app/

I have a question

1. In the first request i get "origin" and "app_guid" and jwt token

In the jwt claim i get a "Subject" which is current user in app. But this is in the form of link https://support.zendesk.com/api/v2/users/1000.json

How can i get this users and its organization detail from zendesk?

I wanted to achieve below:

I wanted to get unique identifier of user/organization, which do not change to store it in my app DB. and next time when i receive the request i can easily figure out if this is already registered user/organization in my app 

How can i achieve this?

Thanks

Shiv