This way works fine for me:
var request = new RestSharp.RestRequest("RESOURCE", RestSharp.Method.POST) { RequestFormat = RestSharp.DataFormat.Json }
.AddBody(BODY);
var response = Client.Execute(request);
// Handle response errors
HandleResponseErrors(response);
if (Errors.Length == 0)
{ }
else
{ }
Hope this helps! (Although it is a bit late)