the newer switches that use the ELS syntax all use the new [forwarding-options dhcp-relay] syntax (EX4300, QFX, EX4600, EX9200).
basically define a server group, set it as active, and define which IP interfaces you want to enable DHCP relay on.
Unless they've fixed it, please please please make sure you include the following command:
set forwarding-options dhcp-relay forward-snooped-clients all-interfaces
we saw without this, if transit DHCP traffic passed the switch (ie from a downstream router sending a relay back to a DHCP server), switches configured with this "new style" would eat any transit DHCP packets, because it seems to snoop any and all DHCP messages once you enable DHCP Relay on an interface. In some versions of code the above command wasn't needed, in others it was, so I just default to configuring it to be safe.
the new method of DHCP relay early on was really buggy, but its been pretty solid as of late.
will