Using fiddler with Windows Authentication

I am testing some proxy settings for our application but I need to test a proxy that requires Windows Authentication (or network credentials).

For testing, I assigned the credential of the proxy to the network credentials

System.Net.WebProxy proxy = new System.Net.WebProxy("127.0.0.1", 8888);
proxy.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;
//proxy.Credentials = new System.Net.NetworkCredential("1", "1");
System.Net.WebRequest.DefaultWebProxy = proxy;

Currently, I use Fiddler with the Require Proxy Authentication rule turned on. How can I configure Fiddler so I can use the Windows credentials instead of the default "1" / "1" credentials?