We could not get the iCal Export URL working so we contacted their support team. This was their reply
" We would like to confirm that our system support ICALs from all platforms, as this is a standardized format for calendar sharing.
However, upon checking the photo/snap shot you’ve shared with us it seems the ICAL you have been trying to add has some extra elements at the end of the URL, which, it makes them incompatible with us. A support ICAL should end on .ICS as the example below:
The snapshot is showing some additional elements at the end.
We kindly invite you to revise the correct option to export with the support department of your external platform, and try adding them again. For further guidance about our ICAL tool, please, make sure to visit the following article:
How can I recertify this problem?