That's one of the things that put me off about boku as I first look at the API for any service a plugin offers first before jumping in to better help me understand expected complexities of adding new features in the future (it was there previously under fortumo's site). I think I even tried reaching out a year ago and they never bothered to follow up on the form signup/contact.
Maybe such documentation is walled-off and only available for registered clients. Let's see.