Hello Stanley,
The Stripe App you publish is associated with your own Stripe account which we'll call a "platform" similar to using Stripe Connect. On that account, you would create a Connect WebhookEndpoint [1] which will listen to the relevant Event type(s) created on all of your connected accounts (which installed your Stripe App). In that case, the Event object [2] will have the `account` property [3] set to the id (acct_123) of the connected account the Event is from. This is covered in our documentation here [4].
Note that this can be complex to test in Test mode. Usually, you're testing your Stripe App on the account that publishes/owns the app. Because of this, the Event(s) it generates happen on your own account so they are not sent to your Connect WebhookEndpoint. To test this properly, you have to publish your Test app and run an external test as documented here [5] so that you can install it on a separate Stripe account (different from your own). Once you have done that, Events from that specific account will have the `account` property set as expected.
Best,
Remi