It seems that custom properties do not generate webhook events. Is this true?
Why do you say that? To my knowledge, you can subscribe to changes on custom properties by defining new properties in your app. See below.
As the Webhooks API Overview documentation notes: "If a customer has no property defined that matches this value, than this subscription will not result in any notifications." This implies that custom properties are supported.
I didn't see any of the custom properties listed in the dropdown list. I assumed that it would not work if I manually entered the internal name of the property, but it actually does work. I don't understand why the custom properties are not listed in the dropdown -- it would be nice if they did.
Thanks for the clarification. No custom properties are listed in the dropdown because custom properties can be portal-specific, so the tool wouldn't know which portal's custom properties to display. For example, how would it choose between your production portal, one of your developer test portals, or one of your customers' portals?
The webhook subscription tool in your app doesn't actually create custom properties through the the Contact Properties API (or any of the other Properties APIs), it just creates webhook subscriptions for properties with the internal names you define. That way, once your app creates the corresponding custom properties in the customer portal, you can begin monitoring events.