The "Subscription" trigger event is available by default on Web push trigger campaigns. Users trigger it when they opt-in to push notifications on your website for the first time.
If your Trigger campaign on the "SUBSCRIPTION" event doesn't have any sent, it's most likely that your SDK is not up to date.
Your WEB SDK version may not be compatible with this feature. This feature is only available since version 3.0 of Batch WEB SDK.
Follow this guide to migrate to the latest version of our WEB SDK.