Custom rules via command line /editor?

[Ed V.]

This is a follow-on to a prior post:
https://forum.opnsense.org/index.php?topic=39145.0

It looks like in order to fully meet https://www.rfc-editor.org/rfc/rfc4890, I need to create specific rules for some ICMPv6 types versus the "wide open everything" rule I have at the moment.

Digging into the available OPNSense and FreeBSD IPFW /PF documentation, this should be possible if I create rules from the command line by editing pf.conf?  ipfw.conf?  ipfw?  pfctl?  Something else?

But where, what syntax (since OPN seems to mix IPFW and PF), how to ensure that it's accepted by the system, etc. is still pretty darn foggy to me.

If my current understanding is correct, somewhere in the file structure I should be able to create a new file /rule something like:

pass in log quick inet6 proto ipv6-icmp all icmp6-type 4 code 2 keep state
pass out log quick inet6 proto ipv6-icmp from (self) to fe80::/10 icmp6-type 4 code 0

along with the other types /codes as listed in RFC4890

But how? And where?

Can anyone point me in the right direction so I don't self-inflict a breakage?