How to be part of the Swift Playgrounds Subscription?

As I understand, you need to produce a .playgroundbook for allowing subscriptions to it, rather than a .playground There’s a lot on information on publishing a feed for subscriptions here: https://developer.apple.com/library/content/documentation/Xcode/Conceptual/swift_playgrounds_doc_format/index.html though it took me a bit to finally be able to share the direct link to my feed file. The url including the prefix as suggested by the docs don’t seem to work for me yet. Maybe missing the url referrer? Haven’t checked that yet, though. Hope the this helps, Marcelo