Can't connect Android device to own openhab cloud instance

With help of the community I was able to set up my own openhab cloud instance. Now I want to connect my phone to this cloud instance. Instructions seems to be pretty easy

but when I set this URL the port 443 is automatically removed (what has no further effects, I guess)

and the device will not show up in the list.

What else needs to be configured to make my phone appear in the device list?

In order to make notifications work I need to install FCM, right?
Needs this to be done before registering a device?

Best regards

Is there anything else that I can provide so that anybody can give me a hint?