Notifications: Providing custom data when using APNs directly

I’m sending push notifications to my standalone expo app by calling FCM and APNs directly (not using the expo notification service).

So far it seems to work great with one exception; I can’t seem to append custom data to the notification in
iOS. (It works fine on android using the body-field in the FCM request.)

In short, what I’m looking for is what fields to use in the APNs API request in order to be able to access the data in the notification event listener. I can’t seem to find any documentation on this anywhere.