How to get customer id via webhook

Basically I want to have a way to programmatically get the XXXXX id from this url: [organization].zendesk.com/agent/users/XXXXX/requested_tickets. As I understand this id is the one that identify the customers on zendesk side.

My idea was to create a webhook that is sent when a new customer is created in zendesk, but I don't see any way in which I can create a trigger under Business rules for such event. Also, when adding the webhook there is option that instead of trigger the webhook is triggered by a zendesk event, but also in that list there doesn't seem to exist this action. Beside this id I also need customer email, but I guess that one is easier to add.