https://android.jlelse.eu/firebase-push-notification-using-curl-command-devoid-backend-e63795d282c4
https://firebase.google.com/docs/cloud-messaging/http-server-ref

curl -X POST --header "Authorization: key=<SERVER KEY>" \
    --Header "Content-Type: application/json" \
    https://fcm.googleapis.com/fcm/send \
    -d "{\"to\":\"<FCM TOKEN>\",\"notification\":{\"body\":\"ENTER YOUR MESSAGE HERE\"}}"



curl -X POST --header "Authorization: key=<Server key>" \
    --Header "Content-Type: application/json" \
    https://fcm.googleapis.com/fcm/send \
    -d "{\"to\":\"<FCM TOKEN/TOPICS>\",\"content_available\":true,\"priority\":\"high\",\"data\":{\"key1\":\"value1\"}}"