using System;

public class Program

{

    public class MessageOf<T>

    {

        public T Message {get;set;}

    }

    public class AppAccess {

        public string Id {get;set;}

    }

    public static void Main()

    {

        var msg = new MessageOf<AppAccess> { Message = new AppAccess { Id = "123" } };

        Console.WriteLine(System.Text.Json.JsonSerializer.Serialize(msg));

    }

}