From what I can recall, you first need to define the DHCP_SEND_HOSTNAME macro. I think you also need to define a DHCP_CLASS_ID macro and finally you need to call sethostname() in your code to set the name you want to use.
if you have a look at the ports\rcm6700\main.c file in the Rabbit BACnet stack at bacrabbit.sourceforge.net you will see how I did it.
Regards,
Peter,