As specified in the "CUSTOMISATION" section of the poudriere man page:

https://fossil.etoilebsd.net/poudriere/doc/trunk/doc/poudriere.8.wiki#x435553544f4d49534154494f4e

poudriere will look for per-port options in a file:

etc/poudriere.d/options/<portname>/options

If poudriere is building net/samba41, then it will look for port-specific
options in:

etc/poudriere.d/options/net_samba41/options

The etc/poudriere.d/options/net_samba41 directory will be nullfs mounted
on the /var/db/ports directory of the jail.  This allows net/samba41 to be
built with custom options.