Variables comes either from one of the following

1. Parent or

...

1. Incoming Webhook Payload

2. Form variables

3. Response Headers

4. Query params

Response Headers

Response headers is available from the parents call to the child call by using {{responseHeaders.myHeader}}

...

Log example. Here we can see available variables like {{responseHeaders.Server}} which would resolve to cloudflare. or {{responseHeaders.X-Frame-Options}} which would resolve to SAMEORIGIN in the example above.