401 Error Consuming REST service from code but works in browser

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service

REST

I am trying to consume a REST service from another company. When I use the link in the browser it works fine. However I get the 401 error when accessing from code. What am I doing wrong? Do I need to add something to the web.config of the web app?

string URL = @"https://hdapps-qa.homedepot.com/MYTHDPassport/rs/identity/isSessionValid?thdsso=TEST&callingProgram=PSC&Submit=Submit";
            HttpWebRequest req = WebRequest.Create(URL)
                                 as HttpWebRequest;
            string result = null;
            req.Method = "GET";
            req.UseDefaultCredentials = true;
            req.PreAuthenticate = false;
            req.Credentials = CredentialCache.DefaultCredentials;
            using (HttpWebResponse resp = req.GetResponse()
                                          as HttpWebResponse)
            {
                StreamReader reader =
                    new StreamReader(resp.GetResponseStream());
                result = reader.ReadToEnd();
            }