using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var ChangedDate = "2/17/21";

        var body = new

        {

            op = "add",

            path = "/fields/Custom.Application_Name",

            value = "2/17"

        };

        string json = JsonConvert.SerializeObject(body);

        Console.WriteLine(json);

    }

}

​