Hello,
The other thread that you found is still pretty accurate at this point. As one reply mentioned, you can automate the ‘transactional’ behavior using some combination of custom fields, event tracking, event start triggers, and personalization tags.
If you are looking to go the API route, I don’t believe this can currently be accomplished via the v3 APIs. However, you can create and send campaigns with the v1 APIs. If you are looking to create and send the campaign immediately, you can use the campaign_create call. If you want to send a previously created (either via the API or in the ActiveCampaign UI) you can use the campaign_send call.
I hope this helps point you in the right direction!
-Matt