Universal Links not working in my Xamarin.Forms iOS app

I followed these instructions, but I can't get my app to respond when clicking on specific web links.

https://xamarinhelp.com/ios-universal-links/

Here is my apple-app-site-association file. It has no extension. The file downloads when I access it with Google Chrome using a link like this. https://portal.mydomain.com/.well-known/apple-app-site-association

{
   "applinks": {
     "apps": [],
     "details": [
       {
         "appID": "MYAPPID.com.mydomain.appname",
         "paths": [ "/PublicOffers/OffersPage" ]
       }
     ]
   }
}

Here are my applink settings.

applinks: portal.mydomain.com

Here are URL examples.

https://portal.mydomain.com/PublicOffers/OffersPage?id=SOMEGUID&cid=SOMEGUID

https://portal.mydomain.com/PublicOffers/OffersPage/SOMEGUID

When in Debug mode, I've noticed that ContinueUserActivity and OpenUrl never get called.

Here is my current development environment. I'm in the process of configuring a newer MacBook Pro, but at this time I'm connecting to an older MacBook running XCode 8.2.1 (can't go any higher).

Microsoft Visual Studio Enterprise 2015
Version 14.0.25422.01 Update 3
Microsoft .NET Framework
Version 4.7.02556

Installed Version: Enterprise

Xamarin 4.3.0.789 (4d2ed3d)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.

Xamarin.iOS 10.4.0.128 (ba11e48)
Visual Studio extension to enable development for Xamarin.iOS.

Any ideas why this is not working? Thanks!