Authentification of clients

Hello,

I have a Nging to Forward Proxy.

I want auto authent clients requests with dedicated account manage by me.

So, clients don't know never this dedicated acccount.
=> Just, they send their requests without an authentication option.

In Nginx, when it receive requests to site1, Nginx auto authenticate request to site1 with dedicated account manage by me in local.

I don't know how to make it ? :(

Thanks for any helps !

Rominou27