Binding to FontAttributesProperty

Hi,
I want to bind to the FontAttributesProperty of a button
I have tried the following

var btn = new Button()
            {
                Text = item.Title,
                TextColor = Color.Black,
                HeightRequest = 40,
            };
          btn.SetBinding(Label.FontAttributesProperty, "FontStyle");

But with no result.

How can I do this?
Do I have to make a custom binding?

Br Morten