Report Server Custom Authentication

I need to implement custom security in power bi report server. I have followed the standard example here:

https://github.com/Microsoft/Reporting-Services/tree/master/CustomSecuritySample

In above example user is redirected to login page; In my case user is already authenticated (SSO) and don't want to redirect user to login page. I want to pass authenticated user credential programitically. 

Thanks,

Anoop agarwal