Get current client username for SingleSignOn

Hi there

I have a asp.net website (Framework 4.5, IIS 8) for local intranet usage.
The server and also the clients are in the same domain.
In there I have implemented a little user management (SQL table based).

Now I need to find a way to just get the current username and domain
who is using the website (not the IIS user).
I have already tried to enable windows authentication, but with
this the website always prompts for a username and password.

Good would be if this variable would be filled:
System.Web.HttpContext.Current.User.Identity.Name

Any help is appreciated

Thanks and Best regards
Frank