login web page redirects automatically and unable to analyze it with Fiddler

I'm trying to use .Net to analyze the response from a URL. The URL will first redirect to a login page for credential normally. But when I'm in corporate network, it seems to submit my domain credential automatically when redirecting to login page and the login page is not really loaded in browser. what is happening? I'm planing to POST my credential in .net code but I can't find the login page session in Fiddler. Maybe there is a way to prevent browser from redirecting automatically? Any ideas are appreciated.