As at Feb 2021 Upvio natively supports Apple calendar syncing.
Simply head to your calendar connections > add icloud calendar connection > go ahead and log into iCloud and create a unique App-specific password
Once you have the app specific password, then enter it into Upvio to commence syncing.
The "What is this" section of the login screen gives you a link to the apple article showing you how to generate an app specific password for iCloud
Once you are connected, please set your timezone and assign whether you want Upvio to just read your Apple calendar or write to it also.