Hi,

The calendar events must be added by the API methods in Javascript since the calendar is rendered and initialized in javascript. In src/js/custom/apps/calendar/calendar.js you can find handleNewEvent function and refer to the below code that adds a new event:

// Add new event to calendar
calendar.addEvent({
 id: uid(),
 title: eventName.value,
 description: eventDescription.value,
 location: eventLocation.value,
 start: startDateTime,
 end: endDateTime,
 allDay: allDayEvent
 });