Hello Stefan,
I see that you already sent a request with the Homey Webhook URL, and your request is approved.
In cases like these, we approve access to the advanced Web API even to users.
Yes, there is documentation for this:
https://developer.nuki.io/page/nuki-web-api-1-5/10/#heading--configure-webhooks
Please refer to section 6. Use case 10 has an example for the webhooks.