public static Uri addCredsToUri( Uri u, string user, string pass )
{
    UriBuilder uriSite = new UriBuilder( u );
    uriSite.UserName = user;
    uriSite.Password = pass;
    return uriSite.Uri;
}

string user = "uuuuuuu";
string pass = "ppppppp";
string authHdr = "Authorization: Basic " + Convert.ToBase64String(Encoding.ASCII.GetBytes(user + ":" + pass)) + "rn";

webBrowserCtl.Navigate("http://example.com", null, null, authHdr);