How to use NotificationBox with Thunderbird?

As the title mentions, I’m trying to use NotificationBox with Thunderbird, but there’s extremely limited documentation for it. The only thing I can find was this: https://developer.thunderbird.net/add-ons/updating/tb102/adapt-to-changes-in-thunderbird-92-102#notificationbox.appendnotification

It shows the following:

let notification = notificationbox.appendNotification(
    notificationId,
    {
        label: notificationLabel,
        priority: notificationbox.PRIORITY_CRITICAL_HIGH,
        eventCallback: eventCallback,
        notificationIs: customElementClass
    },
    buttons
);
notification.messageImage.src = imageUrl;

The issue, is that it does not explain how notificationbox is defined.

See also https://thunderbird.topicbox.com/groups/addons