You can insert your own code into the Portal version, not the AGOL version. Don't they have support for Google? I don't use it, I use Matomo. I wish I could find my notes on it. I think I found the source for the HTML in the builder C:/Program Files/ArcGIS/Portal/apps/webappbuilder/stemapp and inserted my tracker code there. Then it will be inserted each time you create a new app.
