I have added PushNotification plugin to my OpenFire server. But I have no clue on configuring FCM (or any other push notification system) using this.
The official readme does not have much details.
Any help on where I can do the configuration will be appreciated.
PushNotification plugin doesn't have direct integration with FCM as the plugin provides implementation of XEP-0357.
If you have your own server side integration with FCM then by using CallbackOnOffline plugin, you can get http call from Openfire whenever there is an offline message in 1-1 chat. Plugin configuration details are mentioned in it's README file.