Unless things have changed, my recollection is that MS Outlook does not support CalDAV out of the box. Before doing anything else, you will need to find and install a CalDAV plug-in for Outlook. I do not know which one works best these days.
Once you have added CalDAV support, look here for some information that should help add the FastMail calendar:
https://www.fastmail.com/help/techni....html#calendar