it is better to use json after post your resuest like below
var clien = new RestClient("https://smple.com/");
var request = new RestRequest("index", Method.POST);
request.AddHeader("Sign", signinstance);
request.AddJsonBody(JsonConvert.SerializeObject(yourclass));
var response = client.Execute<YourReturnclassSample>(request);
if (response.StatusCode == System.Net.HttpStatusCode.Created)
{
return Ok(response.Content);
}