Webhooks are automated messages sent from your app when an event happens. You can have multiple webhooks associated with a single app.
To add a webhook, click on an existing App. On the overview page, click the "Add webhook" option under the Webhooks section. In the pop-up displayed, enter the webhook URL, click the "Add webhook" button, and it will be successfully added to the list of webhooks.
You can then receive app events on that webhook.