I was able to make the creation event works! Basically, I just had to create a custom property to the event type and use it in the header template. Without a custom property, it was not working (nevertheless, the webservice was not returning any kind of error)
If you’re getting a 204 response and the contact is being created, the most likely issue is that the events are being filtered out of the timeline view. You’ll need to click the Filter timeline button at the top of the timeline, and make sure that your integration is checked.
If you’re still not seeing the event after that, can you message me directly with a link to the contact record that should have the event?
Timeline events will only appear in the newer contact view. You’ll can switch to the new view from the blue box you’ll see in the bottom right. Once you switch, you’ll want to check the timeline filter settings as I mentioned previously.