I found an other solution that work perfectly .
private void sendNotification(String registrationID,String message,String title,String accessToken)
{
HttpWebRequest httpWReq =
(HttpWebRequest)WebRequest.Create("https://api.amazon.com/messaging/registrations/" + registrationID + "/messages");
Encoding encoding = new UTF8Encoding();
string postData = "{\"data\":{\"message\":\""+message+"\",\"title\":\""+title+"\"},\"consolidationKey\":\"Some Key\",\"expiresAfter\":86400}";
byte[] data = encoding.GetBytes(postData);
httpWReq.ProtocolVersion = HttpVersion.Version11;
httpWReq.Method = "POST";
httpWReq.ContentType = "application/json";//charset=UTF-8";
httpWReq.Headers.Add("X-Amzn-Type-Version",
"com.amazon.device.messaging.ADMMessage@1.0");
httpWReq.Headers.Add("X-Amzn-Accept-Type",
"com.amazon.device.messaging.ADMSendResult@1.0");
httpWReq.Headers.Add(HttpRequestHeader.Authorization,
"Bearer " + accessToken);
httpWReq.ContentLength = data.Length;
Stream stream = httpWReq.GetRequestStream();
stream.Write(data, 0, data.Length);
stream.Close();
HttpWebResponse response = (HttpWebResponse)httpWReq.GetResponse();
string s=response.ToString();
StreamReader reader = new StreamReader(response.GetResponseStream());
String jsonresponse = "";
String temp = null;
while ((temp = reader.ReadLine()) != null)
{
jsonresponse += temp;
}
}