Web api call using post by passing soap+xml data and proxy credientials

See more:

C#

REST

WebAPI

WCFServices

HI,
I am trying to invoke api service using post and iam getting excetion like
"

The remote server returned an error: (407) Proxy Authentication Required.

"
My Code: worked in out of office network and not working in my office network.

// Restful service URL
            string url = @"https://url/url";

            var filePath = @"..\PostXML.xml";

            ASCIIEncoding encoding = new ASCIIEncoding();
            string strResult = string.Empty;

            string SampleXml = File.ReadAllText(filePath);
            string postData = SampleXml.ToString();

            byte[] data = encoding.GetBytes(postData);

            HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(url);

            webrequest.Method = "POST";

            webrequest.ContentType = "application/soap+xml";

            webrequest.ContentLength = data.Length;

            webrequest.Headers["Authorization"] = "Basic " + Convert.ToBase64String(Encoding.Default.GetBytes("username:password"));
            webrequest.Headers.Add("Client-Id", "2345678igfdfguio");
            webrequest.Headers.Add("Client-Secret", "490iugfghjki-ltyhjvb");
            Stream newStream = webrequest.GetRequestStream();
            newStream.Write(data, 0, data.Length);
            newStream.Close();

            HttpWebResponse webresponse = (HttpWebResponse)webrequest.GetResponse();

            Encoding enc = System.Text.Encoding.GetEncoding("utf-8");

            StreamReader loResponseStream = new StreamReader(webresponse.GetResponseStream(), enc);

            strResult = loResponseStream.ReadToEnd();

            loResponseStream.Close();

            webresponse.Close();

            strResult = strResult.Replace("</string>", "");


            Console.Write(strResult);

What I have tried:

I tried by passing proxy values with credentials but not helped, please with any sugessions.