How to use a type in attachedBindableProperty in xaml?

public class LayoutHelper<T>
{
     public static readonly BindableProperty ItemSourceProperty = BindableProperty.CreateAttached("ItemSource", typeof(ObservableCollection<T>), typeof(LayoutHelper<T>), null);
....
}

In C# like:

        Extensions.LayoutHelper<string>.SetItemSource(hfl, historickeys); 

But how to use it in xaml:

        <Setter Property="extensions:LayoutHelper<string(this throw a error)>.ItemSource" .../>