A calendar key is a specific portion of a unique, shareable calendar link.
When you open a calendar on a browser, look in the browser bar to view the URL you are using.
An account user URL will have /c/ followed by the calendar id and will end in the calendar title:
Important note: account user URLs do NOT contain a calendar key and cannot be used for setting up Zapier integrations.
A shareable calendar link will have /ks followed by a 16-digit alphanumeric string (the calendar key):
🔑 The calendar key is the portion of the link starting with ks, as underlined in red above.