The DHCP Relay daemon at Services > DHCP Relay will relay DHCP requests between broadcast domains for IPv4 DHCP. For IPv6 clients, use Services > DHCPv6 Relay. Aside from working on different address families, the two services have the same configuration style.
Check Enable, then click or ctrl-click the Interface(s) to relay. Optionally check the box to Append circuit ID and agent ID to requests, and then enter an IP address for the Destination server which is the actual DHCP/DHCPv6 server.
NOTE: Relay may not work as intended from the firewall if the target DHCP server is across an IPsec tunnel. If possible, it is best to enable a DHCP helper on the switch instead.