SO post - http://stackoverflow.com/questions/32384724/myob-oauthservice-gettokensasync-not-showing-dialog/3240807

arl_winforms_sample.cs

using System;
using System.Text.RegularExpressions;
using System.Windows.Forms;
using MYOB.AccountRight.SDK;
using MYOB.AccountRight.SDK.Communication;
using MYOB.AccountRight.SDK.Services;

namespace arl_winforms_sample
{
    public partial class Form1 : Form
    {
        private readonly Uri _authorizeUri = new Uri(
            string.Format("https://secure.myob.com/oauth2/account/authorize?client_id={0}&redirect_uri={1}&scope=CompanyFile&response_type=code",
                Configuration.ClientId, Uri.EscapeDataString(Configuration.RedirectUrl)));

        private readonly Uri _logoffUri = new Uri("https://secure.myob.com/oauth2/account/logoff");
        private readonly SimpleOAuthKeyService _keyService = new SimpleOAuthKeyService();

        public Form1()
        {
            InitializeComponent();
        }

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            webBrowser1.Navigate(_authorizeUri);
        }

        private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            var content = webBrowser1.DocumentText;
            var regex = new Regex(@"\<title\>(.+?)=(.+?)\</title\>");
            var match = regex.Match(content);
            if (!match.Success || match.Groups.Count != 3)
                return;

            switch (match.Groups[1].Value.ToLowerInvariant())
            {
                case "code": // we have a code
                    var code = match.Groups[2].Value;
                    var config = new ApiConfiguration(Configuration.ClientId, Configuration.ClientSecret, Configuration.RedirectUrl);
                    var service = new OAuthService(config, new WebRequestFactory(config));
                    var tokens = service.GetTokensAsync(code).Result; // <= blocking
                    _keyService.OAuthResponse = tokens;
                    break;

                case "error": // user probably said "no thanks"
                    webBrowser1.Navigate(_logoffUri);
                    break;
            }
        }

        
    }

    public class Configuration
    {
        public const string ClientId = "####";
        public const string ClientSecret = "****";
        public const string RedirectUrl = "http://desktop";
    }
}