C# Proxy request.GetResponse()报错 需要代理身份验证，请问要怎么写验证

HttpWebRequest request = (HttpWebRequest)WebRequest.Create(strUrl);
response = request.GetResponse();
stream = response.GetResponseStream();
if (!response.ContentType.ToLower().StartsWith("text/"))
    value = SaveBinaryFile(response, path);

到response = request.GetResponse(); 的时候报错：System.Net.WebException: The remote server returned an error: (407) Proxy Authentication Required.

strURL = http://ac-ay44yffl.clouddn.com/lGVSCnNtEVQLsDRDuswd6JEFEJ32xn99TV8XinYY.jpeg

要在response 前面添加代理吗？ 要怎么写呢 ？