I have written a Google Apps Script which successfully attaches a Google Sheets document to a newly created Calendar Event. The following code shows how this is achieved:
var eventObj = {
summary: eventName,
start: {dateTime: startDate.toISOString()},
end: {dateTime: endDate.toISOString()},
attachments: [{
'fileUrl': 'https://drive.google.com/open?id=' + fileID,
'title' : 'Booking Sheet'
}]
};
Calendar.Events.insert(eventObj, inStorePartiesCalendarID, {'supportsAttachments': true});
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