It was unclear after chatting with Sales/Support. Are custom bahavior events possible without the Marketing Hub Enterprise account? Can I setup custom events with the Events API with the Marketing Hub Professional account?
From my point of view, the answer is yes and no -- it depends on which approach you are planning to take in order to use this feature.
If you’re planning to use HubSpot APIs via “Private App” authentication or custom JavaScript snippets to send event completions, you won’t be able to use this feature without at least 1 Enterprise subscription, as specified here.
However, if you’re planning to use HubSpot APIs via “Public App” authentication (for which you’d need a HubSpot Developer Account), this should be possible. Note that a “Public App” doesn’t necessarily have to be “Public” -- as long as key information relating to the app is kept secret, it can be used in private development contexts.
I hope this proves helpful. Please let me know if you have any follow-up questions.