Given a FreeBSD instance without a configured network interface that you’d like to configure, first check what the name of the interface you want to configure is with ifconfig. (Mine is em0 in this instance.)

Then we need to add the configuration telling services that we want to use DHCP for this interface, and setting up our default router (use your IP, not mine!) too:

cat >> rc.conf <<CONF
ifconfig_em0="DHCP"
default_router="192.168.1.1"
CONF

And then we need to start dhclient on the given interface:

service dhclient start em0

Hey presto, you should see dhclient finding a DHCP server and being handed an IP address for em0.