How to create app.xaml inside a dot net standard 2.0 project?

I see a lot of chit-chat about support for Dot et Standard 2l;0, but no way to implement it.

When I create a new solution using Rider (or Visual Studio), I can create a "normal" pcl with app.xaml.  I don't have the option to create a full Dot Net Standard 2.0 solution.

I can add a new PCL, sure, but that does not cause it to act as the root of the app.  It cannot host app.xaml.

What am I missing?