Great question — answering as the UphillTech team:
GA4 interface is somewhat misleading. It allows you to make GA4 events based on other already tracked GA4 events. It does not allow you to track anything new. Without using GTM or custom scripting (gtag.js) you won't be able to track custom events. GA on it's own is only able to track the most simple things like exit links, downloads, pageviews and scrolls. You could try a workaround. You could check which page appears after successful booking and look at the instances of that pageview, but it's not very reliable and you'd have to know the data pretty well to do that without severe mistakes in analysis.
Full answer by BNazaruk: https://stackoverflow.stackexchange.com/a/78709049 (CC BY-SA 4.0).
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