http://stackoverflow.com/q/16768314/1252290

var request = new RestRequest("some/where", Method.POST); // Create a new request

request.RequestFormat = DataFormat.Json; // The request data is JSON
request.AddBody(parameters); // parameters is a dictionnary object, so it is serialized as a JSON string by Restclient
request.AddHeader("Content-Type", "application/json"); // Make sure that the request is detected as a JSON by the server

string body = request.Parameters.Find(param => param.Type == ParameterType.RequestBody).Value.ToString(); // Get the raw request body

request.Parameters.RemoveAll(param => param.Type == ParameterType.RequestBody); // Remove this body from the request

// Perform some operation to this raw body

// Adding back the modified body to the request
request.RequestFormat = DataFormat.Json;
request.AddParameter("application/json", body, ParameterType.RequestBody);