Doing something with the success response from a Webhook Craft Webhooks is great for sending one POST to a service... but if you need to parse the response from that hook... is there a way to do that? I can't see it as any option in the plugin, and I can't see a Service or anything for having Craft listen to that event?

I'm trying to get it so when a new User is created it's also subscribed to an Active Campaign account. I've no problem creating the Contact in Active Campaign through the Webhook plugin - it works flawlessly. But you can't then add the new Contact to an Active Campaign List because that requires a second POST that includes the new ContactID and a pre-defined List ID.

Craft has no clue about the new ContactID because it wasn't listening or parsing the response to the first successful Webhook (which contains an object with that data in it).

Likewise, it doesn't look as though it's possible to perform both actions in one API call.
You might want to look into creating a simple Craft plugin to do this. You can make the first call to add the contact and get the ID in response. Then use that ID to add to a campaign.

Do not be intimidated by plugin development, your problem is fairly easy to solve. If you are on Mac, I like the Paw app to test API calls.