http://3m2pgmhd3gcmlj2m27oe6wcm25wyunmqt3lumdltmqfn2bvlept6exid.onion/en/api
You just need to fill the iCalendar feed into the 'FEED URL' box. If your events do not have geolocation you will also want to select the default 'Offline' location for events. Outgoing Each group has an link to it's upcoming events feed on the group page.