OneSignal App Integration for Web Push Notifications

Kia ora - I see there is an integration with OneSignal enabled in Retool. I'm hoping to use this to send users web push notifications.

I'm trying to configure accordingly, having set up a OneSignal account ready - but am not able to get past the point of subscribing the user, which requires incorporating the OneSignal SDK into the app page. It's obviously possible somehow, given the Retool Integration, but there is no documentation anywhere on how to achieve this, and OneSignal don't know how to accomplish this in the Retool platform.

The difficulty is that a snippet has to be incorporated into the of the page, to call the OneSignal SDK. There doesn't seem to be an effective way of doing this (as discussed elsewhere for similar applications). I've tried via a custom component, html component, loading the SDK as a library, or running it in a JavaScript snippet. Each option fails for different reasons! What do you advise at the correct approach?

With grateful thanks