using System;

using System.Text.Json;

using System.Text.Json.Serialization;

using System.Web;

public class Program

{

    public static void Main()

    {

        // Client

        var foo = new AuthEndPointArgs { ResponseType = "code", ClientId = "a:b" };

        var json = JsonSerializer.Serialize(foo);

        var payload = HttpUtility.UrlEncode(json);

        // Server

        json = HttpUtility.UrlDecode(payload);

        var recvd = JsonSerializer.Deserialize<AuthEndPointArgs>(json);

        Console.WriteLine(recvd.ResponseType);

        Console.WriteLine(recvd.ClientId);

    }

    class AuthEndPointArgs

    {

        [JsonPropertyName("response_type")]

        public string ResponseType { get; set; }

        [JsonPropertyName("client_id")]

        public string ClientId { get; set; }

    }

}