Where can you enter events in the connect app to the calendar? I can deside the option to show only events but don't know how to enter them

As far as I know, there is no interface for events in Garmin Connect, but you can add events on Google or Office365 calendars, if you have them linked with Garmin Connect.

... but if you really want to add an event in Garmin Connect, there is a way, in fact, too: create a Workout (may be a true workout, or just a dummy one), name it in the way you wish to have for your event (i.e. "Sunday Slug Race"), add notes if needed, and schedule it.