Nancy Auth Cookie, bit of a hack but works and uses the least amount of code.

BrowserContextExtenstions.cs

using System;
using System.Collections.Generic;
using Nancy.Authentication.Forms;
using Nancy.Testing;

namespace Nancy.Authentication.Forms
{
    public static class BrowserContextExtenstions
    {
        public static void FormsAuth(this BrowserContext browserContext, Guid userIdentifier)
        {
            var response = FormsAuthentication.UserLoggedInResponse(userIdentifier);

            var authCookie = response.Cookies[0];

            browserContext.Cookie(authCookie.Name, authCookie.Value);
        }
    }
}