@ortin The nuki-bridge-api which is also the basis for pimatic-nuki contains a programming example for adding callbacks.
In my opinion using a callback mechanism isn’t a great concept in this context as the application needs to provide a transport endpoint to have the nuki bridge establishing HTTP connection to your application to send a status update. It would be much better to use a messaging middleware, e.g. MQTT, or a bidirectional protocol, e.g. WebSocket, at this point.
EDIT: Just had a look at what the Nuki guys are up to
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law