The blog post Loki 0.4 Stable Release! mentions:
Calendar in Loki introduces natural language creation for events. For example, type “Meeting with Dan tomorrow at 8 at Starbucks” and Calendar will pre-fill the event title, participant, date, time, and location tabs with the relevant information.
But I haven't been able to find where I can type this. Slingshot simply indicates "No results", there doesn't seem to be a new field or button in the Calender app itself and there is no button for this in the Calendar Wingpanel integration .
I'm using Using Elementary OS 0.4 Loki.