Use the RegisterUser Client Action to make the user subscribe to the notifications.

The PlayerId (OneSignal User Id) will be automatically saved in UserNotificationInfo table.

The PlayerId is needed to send notifications specifically to the user.

If you change the module name, make sure to change the ModuleName Site Property.

Configue your OneSignal app as a Web app and get this info:

Save the appId and safari_web_id in the Site Properties with similar name.

The API Key can be retrieved in the Settings > Keys & IDs. It will be needed to send notification.

 To send notifications you can use the OneSignal Notification Plugin.