var uri = new Uri(url);
if (uri.Query.Length > 0)
{
    var querys = uri.Query;
    if (querys.Contains("username") && querys.Contains("password"))
    {
        var userName = querys.Split('&').SkipWhile(x => !x.StartsWith("username")).First().Split('=').Last();
        var password = querys.Split('&').SkipWhile(x => !x.StartsWith("password")).First().Split('=').Last();
        var type = querys.Contains("type") ? querys.Split('&').SkipWhile(x => !x.StartsWith("type")).First().Split('=').Last() : "Digest";

        cache = new CredentialCache { { uri, type, new NetworkCredential(userName, password) } };
    }
}

http://192.168.2.33/cgi-bin/video.cgi?msubmenu=jpg&username=admin&password=4321&type=Digest

var query =
    uri
        .Query
        .Split('&')
        .Select(x => x.Split('='))
        .ToDictionary(
            x => x[0].TrimStart('?'),
            x => x.ElementAtOrDefault(1)
        );