Клиент C# REST API Prestashop

Я мог ПОЛУЧИТЬ данные Xml из Prestashop API, но не мог PUT/POST данных Xml в PrestaShop API.

Может ли кто-нибудь подсказать, где я могу ошибаться?

public POST_xml()
{
     Uri address = new Uri("http://.../api/countries/1");
     HttpWebRequest request = WebRequest.Create("http://.../api/countries/1") as HttpWebRequest;

     NetworkCredential("15PJQ4V8CXI22JVW1TKZASDF0OAYNBLA", "");

     // Create the web request  
     request = WebRequest.Create(address) as HttpWebRequest;

     // Set type to POST  
     request.Method = "POST";
     request.ContentType = "application/x-www-form-urlencoded";

     // Create the data we want to send.  

     string context = "<prestashop><country><id>1</id><id_zone xlink:href=\"http://.../api/zones/1\">";
     context += "1</id_zone><id_currency/><iso_code>DE</iso_code><call_prefix>49</call_prefix><active>1</active><contains_states>0</contains_states><need_identification_number>0</need_identification_number><need_zip_code>1</need_zip_code><zip_code_format>NNNNN</zip_code_format><display_tax_label>1</display_tax_label><name><language";
     context += " id=\"6\" xlink:href=\"http://.../api/languages/6\">Germanyxx</language></name></country></prestashop>";


     StringBuilder data = new StringBuilder();
     data.Append("&context=" + HttpUtility.UrlEncode(context));


     // Create a byte array of the data we want to send  
     byte[] byteData = UTF8Encoding.UTF8.GetBytes(data.ToString());

     // Set the content length in the request headers  
     request.ContentLength = byteData.Length;

     // Write data  
     using (Stream postStream = request.GetRequestStream())
     {
         postStream.Write(byteData, 0, byteData.Length);
     }
}