HttpClient myClient = new HttpClient();
myClient.BaseAddress = new Uri(URL);
ベース アドレスはクライアントの URL で既に指定されています。
var encodedObject = JsonConvert.SerializeObject(Obj);
myClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
HttpResponseMessage response = await myClient.PostAsync("test.php/yourAPI",new StringContent(encodedObject, System.Text.Encoding.UTF8, "application/json"));
if (response.IsSuccessStatusCode)
{
var responseContent = response.ToString();
var responsebody = response.Content.ToString();
Stream receiveStream = response.GetResponseStream();
string responseBodyAsText = response.Content.ReadAsStringAsync().Result;
}