gistfile1.txt

using System;
using Newtonsoft.Json;

public class MainModel
{
    [JsonProperty(PropertyName = "pageTitle")]
    public string PageTitle { get; set; }

    [JsonProperty(PropertyName = "helpBtnText")]
    public string HelpButtonText { get; set; }

    [JsonProperty(PropertyName = "nestedModel")]
    public NestedViewModel NestedViewModel { get; set; }
}

public class NestedViewModel
{
    [JsonProperty(PropertyName = "headerText")]
    public string HelpButtonText { get; set; }

    [JsonProperty(PropertyName = "submitBtnText")]
    public string SubmitButtonText { get; set; }
}

public static class Program
{
    static void Main()
    {
        var model = new MainModel
        {
            PageTitle = "page title",
            HelpButtonText = "help button text",
            NestedViewModel = new NestedViewModel
            {
                HelpButtonText = "nested help",
                SubmitButtonText = "nested submit"
            }
        };
        Console.WriteLine(JsonConvert.SerializeObject(model));
    }
}