Something like the following should work:
The "--view" arg specifies that it is a calendar folder, and the "--flags" arg with "#" means it will be checked by default in the UI.
prov> sm firstname.lastname@example.org
mailbox: email@example.com, size: 7.88 KB, messages: 2, unread: 2
mbox firstname.lastname@example.org> createFolder --view appointment --color blue --flags "#" /Calendar2
mbox email@example.com> postRestUrl /Calendar2 /tmp/cal.ics
mbox firstname.lastname@example.org> exit
The CreateFolder docs in soap.txt are a good place to poke around in.