You need to pull the user name from Active Directory(AD). Then use it to auto-login to your applicaiton.
Since this is a webpage with AD running and IIS is set to allow it Windows authentication, then you get the user's domain and id a few ways. You have the Page.Identity where you can get it or you can get it form the HttpContext if you builda user class that handles all of this for you, which is what I have. I created a class library with all of this so that I can just include it my project and always have access to the user object and automatically authenticated through AD and expose all of the user properties that I need.
Here's an example, you can use something like the following. Play with this code and take a look at the objects and what they contain. If you are not gettting anything from these, that means that IIS is blocking it. Also, if you are putting this code in the web page, you will probalby need to change HTTPContext to Page.Identity. But this should at least point you in the right direction and I think pushing it into another class is probalby the better approached, so I have include that type of code below.
var id = HttpContext.Current.User.Identity;
var domain = HttpContext.Current.User.Identity.Name.Split("\"c)(0);
var username = HttpContext.Current.User.Identity.Name.Split("\"c)(1);