The "registration" hook is (on all supported platforms) ALWAYs fired, when the app starts up. Since it is hard to figure out if the app runs first time, it does not really make sense to have POST and PUT.

We agreed on having POST only - if the token already exists, for that particular Variant, we simply perform an update in the database.

While this may not be 100% compliant with a PhD thesis, but it's a pragmatic way...