XAML: How do you pass a control/view reference into a behavior?

I am trying to write a compare validator as a behavior. I have the behavior attached to an Entry but I want to pass a reference to the control to compare to into it in XAML. I created a BindableProperty of type Entry and tried using {Binding myCompareToControlName} but it is always null.

Can someone post an example of how this is done?

Thanks,
Jason