I have a site which is built in ASP.net and C#. Let's call it webapp. it uses a Form system to log on into it, and cannot be changed easliy.

I got a request to change the log in to some kind of windows authentication. I'll explain.
Our windows login uses active directory for users to log into their windows account. their login name is sXXXXXXX. X are numbers.
in my webapp, I want to take the users numbers from their active directory login, and check if those exist in the webapp database. if it exists, they will automatically log in. If it doesn't, they will be referred to the regular login page for the webapp system which is currently in use.

I tried changing my IIS to disable anonymous login and enabling windows authentication, therefore making the user browser to send it's current logged in user name to my webapp. I changed the web config as well from Forms to Windows, which made my whole webapp obsolete as the whole forms system did not work.

My question is this - is there a different way for the browser only to send the username to my webapp? I thought maybe javascript, I just don't know how to implement that, if it's even possible. I know it's not very secure, but all this platform and system is built outside the internet, it's on a private network.