Custom routes rendering with Html.BeginRouteForm

Question

I'm using custom route mapping in following way:

    context.MapRoute(
        "CustomRoute",
        "Area/{controller}/{id}",
        new { controller = "Task", action = "Index", id = UrlParameter.Optional },
        new { id = @"\d+", httpMethod = new HttpMethodConstraint("GET") }
    );

With above route I'm going to skip action name, pass parameter via slash /, so I expect to have URL in following way: domain.com/Area/Task/982

It works good by entering URL manually. Now trying to do the same via @Html.BeginRouteForm helper:

    @using (Html.BeginRouteForm("CustomRoute", new { id = Model.ID }, FormMethod.Get))
    {
      @Html.TextBoxFor(m => m.ID)
      <button class="btn btn-default btn-sm" type="submit">Get data</button>
    }

Submitting form it is rendering url in a way: domain.com/Area/Task?ID=982

What is missing above? How can I achieve desired custom route?