Notification
From Admin Panel go to Config > tab Notification
FCM (Firebase Cloud Messaging)

To make notification works you need to have FCM Key. Here the tutorial http://blog.dream-space.web.id/?p=116
please change FCM Key with yours
One Signal
Follow this article to get One Signal App ID & One Signal Rest Key https://documentation.onesignal.com/docs/accounts-and-keys#section-app-id