using RestSharp;


var options = new RestClientOptions("https://api.sandbox.{id}.gr4vy.app/checkout/sessions/{session_id}/fields");
var client = new RestClient(options);
var request = new RestRequest("");
request.AddHeader("Authorization", "Bearer <token>");
request.AddJsonBody("{\n  \"payment_method\": {\n    \"number\": \"<string>\",\n    \"expiration_date\": \"<string>\",\n    \"method\": \"card\",\n    \"security_code\": \"123\"\n  }\n}", false);
var response = await client.PutAsync(request);

Console.WriteLine("{0}", response.Content);

{
  "type": "error",
  "code": "bad_request",
  "status": 400,
  "message": "Generic error",
  "details": []
}