You can bind to an ancestor with a RelativeSource.
Change this line
to this
Don't forget to add the xml namespace for viewModel on top of the file:
xmlns:viewModel="clr-namespace:YourProject.ViewModels"
You can read all about RelativeSource bindings here:
https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/data-binding/relative-bindings#bind-to-an-ancestor