How to use this plugin?

OneSignal is a service that enables push notifications, abstracting details like the platform the device is running on. To use the plugin, you'll first need to configure OneSignal for each of the mobile platforms it will be working on. Once your configuration is done, you’ll have an Application ID and a REST API KEY from OneSignal.

Check how to use Push Notifications with OneSignal.

You can also download the OneSignal Plugin Sample.