using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var model = new ConfigInputModel{Requester = "Requester", Subject = "Subject", Description = "Description"};

        var json = new {

            request = new {

                requester = new {

                    email_id = model.Requester

                }

            },

            subject = model.Subject,

            description = model.Description

        };

        Console.WriteLine(JsonConvert.SerializeObject(json));

    }

}

​

public class ConfigInputModel

{

    public string Requester {get;set;}

    public string Subject {get;set;}

    public string Description {get;set;}

}