Retrieve the windows login user using Forms authentication

I'm working on an ASP.NET application running on our intranet. We are using forms authentication, however the usernames set up will be the same as the Active Directory usernames. I'm trying to find a way to pre-populate the login username field with the Windows username, but can't find a way to access the current Windows user identity.

System.Environment.UserDomainName

System.Security.Principal.WindowsPrincipal.Current.Identity

HttpContext.Request.LogonUserIdentity.Name

I want to authenticate the Windows user or login to the website . Is there any way to do this whilst using Forms Authentication rather than Windows Authentication?