This small post might save time to some other people: it seems like while you can get the current NFC device when your app is launched as a share target, most of the features are actually disabled (you won’t be notified when an NFC tag comes in range and your attempts to post data to the tag will fail).

If you want to create a Share To NFC solution, you’ll need to open you app in full screen (for example using an URI) and perform the operation while you’re the main app on the screen. Your app has to be full screen to use NFC properly.