UpStream Server - Session and Form based Authentication

Hi,
I have upstream/backend webServer which exposes REST API using session based authentication where in for first request it authenticate using form based authentication ( i.e post request and username and password in body) , on success it return session cookie and for next transaction request session cookie needs to be passed.
Can some one pls help me how to implement the same using NGINX as reverse proxy . What code or configuration to have n NGINX ?
Note - Focus here is on UpStream/back end service authentication and not front end..

Regards
Rakhi