Hey, feel free to ask me if you have explicit questions. I dont have much time but will try to answer as soon as possible. First would be that you make your app ready to receive Push Notifications described in the guide in the docs and then you can send them with a http request similar to Google Firebase. The json fields just need to get other names and thats it.
Btw in the openstore you find the app PushClient where you cann see a minimal working example to use the UBports Push Service