c#远程服务器返回错误: (403) 已禁止。

调用一个HTTP接口时返回“远程服务器返回错误: (403) 已禁止。”

请问有哪位大神知道是什么问题，下边是代码

public static string HttpPostFrom(string url, string data, string clientid, string clientsecret)
        {
            string htmlAll = "";
            try
            {
                string SendMessageAddress = url;//请求链接   
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(SendMessageAddress);
                request.Method = "POST";
                request.AllowAutoRedirect = true;
                request.Timeout = 20 * 1000;
                request.ContentType = "application/x-www-form-urlencoded";
                //request.Headers.Add("Authorization","Basic SDIyMDc4MTAwMDMzOjEyMzQ1Ng==");
                string usernamePassword = clientid + ":" + clientsecret;
                request.H