Is there any way to force my phone (pixel, Android 8.0) to open url links to the Washington Post with the Washington Post app? This kind of desired behavior works automatically with other apps/urls for me like the New York Times and Google Maps, but for some reason not with the Washington Post.
This behavior is decided by the app developer, if Washington Post doesn't let such action, then you won't be able to do it.
If they do allow it, then usually the first time you press on a link it will ask you how you want to open it. So it's possible that you have selected the browser as the default. To change that, go to settings>apps>[name of your browser]>defaults>clear defaults