First of all follow the instuctions in the below link, then go to the calender in Office 365 (web) and right click on the 3 dots next to the calender you want to display on your Mirror, click the share and permissions invitation, there are a few opions like, can view when busy, can view all details etc etc, pick the one that suits you then email yourself the share link, copy link and paste into the config.js file under the module “calender-check”
url: https://outlook.office365.com/owa/calender/followed by loads of numbers and your user account ending in/reachcalender.ics
Mine works a treat