Implementing a Floating Action Button

I'm trying to implement a Floating Action Button (FAB) on Android using a custom renderer.

I followed this article:

http://chrisriesgo.com/material-design-fab-in-xamarin-forms/

and have a button appearing but cannot link it to a ListView.

The code required in the custom renderer is

fab.AttachToListView(listView)

but this requires an AbsListView.

How do I get the android native listview in order to pass it to the fab?