Just to add it here for possible migration from OH2 to 3.
OH 3 has no classic UI anymore.
However receiving the command is working also using Basic UI in OH3:
http://host:8080/basicui/CMD?Klingelempfaenger=ON
(Basic UI installation required)
Another way maybe using httplistener:
https://community.openhab.org/t/httplistener-new-very-simple-binding-for-listening-incomming-http-requests/123597