Need to capture field and values from a response message to an API call

I am working on a B2B integration between Zendesk and ServiceNow for incident tickets based on API calls. 

When certain conditions are fulfilled for an incident ticket in Zendesk, a corresponding ticket needs to be created in ServiceNow. This is done based on a Trigger in Zendesk with defined Conditions, and an Action that posts a JSON body through an active Webhook holding a URL endpoint to the ServiceNow system. This works fine as such with the ticket creation, but I need to know the corresponding ticket number in ServiceNow in order to be able to later on update the ServiceNow ticket.

The ServiceNow ticket number can be found in the response message to the API call (can be seen when creating the ticket using Postman), but I haven't found any way/mechanism in Zendesk to capture the ServiceNow ticket number as part of the response message, which also have been confirmed by your/Zendesk support, that this functionality doesn't exist.