This article describes how to share your mailbox.org calendar as ical
Go to the calender that you want to export and create a sharing link.
If you copy the link it looks something like this:
Please add "?ical=true" to this link. With this enhanced link the recipients of this link are able to add it as ical.
Your final ical link would look like this:
For further information, please see the Open Xchange documentation:
Please note that with an ical link the recipient may only read your calendar, not edit it.