Domoticz

https://pushed.co

Free to use. Easier to register and setup, than IFTTT Maker (Webhooks). Clean UI.
Up to 2 free channels (sets of source info to subscribe).

I use it with HomeBridge, but have to hard-code text strings in config.
Domoticz will benefit from that one.

Currently, I use PushOver, but it require licensing for each AppleID.


P.S. They offer free Enterprise plan for open-source projects.. you might skip registration for end-users completely.

Solved by myself.

Settings - Notifications - User HTTP/Actions:

URL/Action: https://api.pushed.co/1/push

POST Data:
{"app_key":"#FIELD1",
"app_secret":"#FIELD2",
"target_type":"app",
"content":"#MESSAGE"}
POST Content-Type: application/json