Forms Authentication and pass through

I have a simple logon page that autheticates to our active directory sever.

What i want is for users who are logged onto the domain to skip this without the need to enter a user name or password.

i have read the following but im struggling to understand where i need to start:

Impersonating by Using LogonUser

If your users have Windows domain accounts, but you are using custom authentication, such as forms authentication, IIS does not authenticate the caller and it passes a logon token that represents the anonymous user account to ASP.NET. To impersonate the caller in this instance, you must programmatically create a WindowsIdentity object for the caller, which you can use to impersonate. Create a WindowsIdentity object either by using a logon token returned from the Win32 LogonUser API, or by using the WindowsIdentity(userPrincipalName) constructor that takes a single parameter of a user principal name (UPN). T ...