Based on the examples in your documentation, it appears that SfBackdropPage is utilized like this as the BASE like this:
<?xml version="1.0" encoding="UTF-8"?>
<backdrop:SfBackdropPage
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:backdrop="clr-namespace:Syncfusion.XForms.Backdrop;assembly=Syncfusion.SfBackdrop.XForms"
x:Class="BackdropGettingStarted.BackdropSamplePage"
Title="Menu">
</backdrop:SfBackdropPage>
But I have the desire to do something like this by keeping my BASE_PAGE components rather than using Backdrop as the base:
<?xml version="1.0" encoding="UTF-8"?>
<hthbase:SingleSelectionBasePage
xmlns:pages="clr-namespace:HthApp.Pages"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:hthbase="clr-namespace:HthApp.Pages.BaseExtensions">
<StackLayout />
<backdrop:SfBackdropPage>
//Content for Backdrop Page goes here
</backdrop:SfBackdropPage>
</hthbase:SingleSelectionBasePage>