The Kea plugin help is obsolete and does not match with firewall GUI.
Please send a link and example, it should be rather up to date:
https://github.com/opnsense/docs/blob/master/source/manual/kea.rst?plain=1
My bad, Monviech. I missed a few options that weren't obvious at first glance. Sorry about that.
It's okay, if theres still issues it cab always be improved. PRs welcome.