Bootstrapper.cs

protected override void RequestStartup(TinyIoCContainer container, IPipelines pipelines, NancyContext context)
{
    base.RequestStartup(container, pipelines, context);

    var formsAuthConfiguration =
        new FormsAuthenticationConfiguration()
        {
            DisableRedirect = true,
            UserMapper = container.Resolve<IUserMapper>()
        };

    FormsAuthentication.Enable(pipelines, formsAuthConfiguration);
}

login.cs

Post["/login"] = _ =>
{
    var loginParams = this.Bind<LoginParams>();
    var user = _userService.Get(loginParams.Username, loginParams.Password);
    if (user == null)
    {
        return HttpStatusCode.Unauthorized;
    }
    
    var authResult = this.LoginAndRedirect(user.Guid);                
    return Response.AsJson(new
    {
        username = user.Username
    }).WithCookie(authResult.Cookies.First());
};

logout.cs

Get["/logout"] = _ =>
{             
    this.RequiresAuthentication();

    var response = this.LogoutWithoutRedirect();
    return Response.AsText("logout").WithCookie(response.Cookies.First());
};