using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var model = new RootObject()

        {

            Prospects = new Dictionary<string, NameModel>()

            {

                { "1234", new NameModel() { FirstName = "Sam", LastName = "Test" }},

                { "some@email.com", new NameModel() { FirstName = "Sue", LastName = "Test" }},

                { "some.other@email.com", new NameModel() { FirstName = "Frank", LastName = "Test" }},

            }

        };

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

    }

}

​

public class RootObject

{

    [JsonProperty("prospects")]

    public Dictionary<string, NameModel> Prospects { get; set; }

}

​

public class NameModel

{

    [JsonProperty("firstName")]

    public string FirstName { get; set; }

    [JsonProperty("lastName")]

    public string LastName { get; set; }

}