You don’t have to serialize the body yourself. Just do
request.RequestFormat = DataFormat.Json;
request.AddJsonBody(new { A = "foo", B = "bar" }); // Anonymous type object is converted to Json body
If you just want POST params instead (which would still map to your model and is a lot more efficient since there’s no serialization to JSON) do this:
request.AddParameter("A", "foo");
request.AddParameter("B", "bar");