To use Newtonson JSON Converter with RestSharp

JsonNetSerializer.cs

using RestSharp.Deserializers;
using RestSharp.Serializers;

namespace RestSharp.Serialization
{
    public interface IRestSerializer : ISerializer, IDeserializer
    {
        string[] SupportedContentTypes { get; }
        DataFormat DataFormat { get; }

        string Serialize(Parameter parameter);
    }
}

public class JsonNetSerializer : IRestSerializer
{
    public string Serialize(object obj) =>
        JsonConvert.SerializeObject(obj);

    public string Serialize(Parameter bodyParameter) =>
        JsonConvert.SerializeObject(bodyParameter.Value);

    public T Deserialize<T>(IRestResponse response) =>
        JsonConvert.DeserializeObject<T>(response.Content);

    public string[] SupportedContentTypes { get; } =
    {
        "application/json", "text/json", "text/x-json", "text/javascript", "*+json"
    };

    public string ContentType { get; set; } = "application/json";

    public DataFormat DataFormat { get; } = DataFormat.Json;
}