I resolved the problem like this
HttpClient Client= new HttpClient();
Client.DefaultRequestHeaders.Add("accept", "Application/JSON");
//Add the content body (which is a json object)
HttpContent content = new StringContent(jsonString);
//Add the header
content.Headers.TryAddWithoutValidation("Content-Type", "application/json");
HttpResponseMessage response = await Client.PostAsync(new Uri(string), content);
response.EnsureSuccessStatusCode();
string ch = await response.Content.ReadAsStringAsync();