I asked this question on the blog post announcing the release of Field Maps (Announcing ArcGIS Field Maps), but when I refreshed the page, my comment disappeared so I'm not sure if it got posted or not, so I will ask here. Plus, I'm assuming other people might have this question as well.
Per the Field Maps documentation (Requirements—ArcGIS Field Maps | Documentation), it says that if you are running ArcGIS Enterprise 10.8 or earlier, you need to register the Field Maps app ID with Portal. However, the URL provided links to a technical support article about how to register Collector for Windows.
Is there any documentation on how to register Field Maps for iOS and Android?