Hey @SimonK
Thanks for asking the Klaviyo Community for help with creating custom events or profile properties.
Unfortunately, there's no easy way to get those properties to sync with Klaviyo. You'd have to send us a custom event via our Track API to includes those custom properties in the event or, if you'd want to reference those properties in a campaign email rather than a flow email triggered by the custom metric, then you'd have to sync those properties as custom profile properties via our Identify API. Check out these other community post that also talk about creating custom events and profile properties. The first link has many additional resources I know you will want to review!
If you find that you need some help building this, check out Klaviyo Upwork, a new freelance marketplace that might be excellent for a small project like this!
I hope this helps! Thanks again for being part of the Klaviyo Community!