How to add some RouteData for MapMvcAttributeRoutes?

Question

I'm building a multi-tenant website and I use both MapMvcAttributeRoutes and the standard MapRoute in my application. I also use this code to get the subdomain name successfully when I'm in a controller that doesn't use the RouteAttribute. I can't get the subdomain value from any controllers/actions that use the RouteAttribute. The value will not exist from the RouteData collection. So how to get the subdomain value when I'm inside an Action that use a RouteAttribute??

Here the code of the RegisterRoutes:

public static void RegisterRoutes(RouteCollection routes)
{
    const string mainNamespace = "Presentation.Website.Controllers";
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
    routes.MapMvcAttributeRoutes(); // Activate attribute Routing

    // This will add the parameter "subdomain" to the route parameters
    // TODO: Do the same for MapMvcAttribute! but how... ?
    routes.Add(new SubdomainConfig(new[] { "www", "yourdomain", "mail" }, new[] { mainNamespace }, CoreSettings.SubDomainKey));

    routes.MapRoute(name: "Default",
                    url: "{controller}/{action}/{id}",
                    defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional },
                    namespaces: new[] { mainNamespace });
}

e.g

This will work:

[AllowAnonymous]
public ActionResult Register(string subdomain)
{
    ViewBag.Subdomain = subdomain;
    var vModel = new RegisterViewModel();
    return View(vModel);
}

This won't work:

[Route("login"), AllowAnonymous]
public ActionResult Login(string returnUrl, string subdomain)
{
    ViewBag.Subdomain = subdomain; // <----------- Empty
    ViewBag.ReturnUrl = returnUrl;
    return View();
}

David