Hey Darren,
Looks like you can use API Trigger Properties in the Webhook URL field. And interestingly, since the URL field is a long text input, you can do all sorts of things using Liquid (fun!). How does the second screenshot look as a potential solution?
If reporting is important and you need to use two separate message steps, then you should be able to apply the exact same process/logic that I wrote in this blog below. We use the User Update step to log the necessary data as a custom attribute, which you can then use to set up the Audience Path to differentiate the Webhook URLs.
https://www.blog.fornowmarketing.com/blog/braze-step-by-step-guide-split-users-based-on-canvas-entry-property
Lmk if you have any questions!
Allan