This is How to Send Notifications from Wp to Visitors device:
so if you were searching for how to integrate push notifications in your wordpress website free service and enable onesignal check the steps:
1. create an account in onesignal push notification website.
2. Download onesignal plugin.
3. put your onesignal.
4. complete the plugin configuration.
i recommend you to watch the video in this post for complete overview.
you may need a vpn to open onesignal website.