FreeBSD's builtin DHCP client (dhclient) supports acquiring IPv6 routes and SLAAC addresses only. It does not support acquiring a DHCPv6 assigned IA_NA address, as it is not officially part of the IPv6 standards.
This article details how to configure and run the net/dhcpcd port to acquire both an IPv4 and an IPv6 address via DHCP servers.
Install net/dhcpcd using your preferred method.
By default, dhcpcd will acquire an IPv4 address lease via DHCP, solicit for an IPv6 router, and generate a SLAAC private IPv6 address.
To configure dhcpcd to also request a DHCPv6 IA_NA address, edit /usr/local/etc/dhcpcd.conf
hostname ia_na 1
Test it by executing dhcpcd:dhcpcd
Edit /etc/rc.conf to enable and trigger dhcpcd at boot time.
dhclient_program="/usr/local/sbin/dhcpcd" ifconfig_em0="DHCP"