Add notes, an URL or files to events in Calendar on Mac
You can add notes to events you create — a list of agenda items, for example. If you add a phone number to the notes, people can call it using their iPhone or Mac (if it’s set up to make phone calls). You can also add a URL to the event, and you can attach files, such as a form for an appointment or a document to discuss at a meeting.
In the Calendar app on your Mac, double-click or force-click an event.
Click “Add Notes, URL or Attachments”.
Do any of the following:
Add notes: Click Add Notes, then enter your note.
Add a URL: Click URL, then enter a web address or other URL.
Attach a file to an event: Click Attachments, then select a file to attach. You can also drag a file to the event info window.
Preview an attached file: Select the file, then press the Space bar.
Remove an attached file: Select the file, then press the Delete key.