using (var client = new HttpClient())
{
 client.BaseAddress = new Uri(Url);
 client.DefaultRequestHeaders.Accept.Clear();
 client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));

 using (var task = client.PostAsJsonAsync(Url, body))
 {
    if (task.Result.StatusCode != HttpStatusCode.OK)
       throw new Exception(task.Result.ReasonPhrase);
 }

}

Nicht sicher, warum wir die bekommen Normalerweise ist nur eine Verwendung jeder Socket-Adresse (Protokoll / Netzwerkadresse / Port) erlaubt xx.xxx.xxx.xx: 80 error

System.AggregateException: One or more errors occurred. ---> System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: Only one usage of each socket address (protocol/network address/port) is normally permitted xx.xx.xx.xx:80
at System.Net.Sockets.Socket.EndConnect(IAsyncResult asyncResult)
   at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state, IAsyncResult asyncResult, Exception& exception)
   --- End of inner exception stack trace ---
   at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)
   at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)
   --- End of inner exception stack trace ---
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)