DHCP option 43 for the ACS URL. You probably want to do option 121 as well to add a static classless route back to your ACS server.
@tiger4020, depending on what your equipment supports, ideally you want to configure your customer prem equipment with two network interfaces. One for PPPoE, and one management. Our management network is DHCP and we use option 43 and 121.
Maintaining two interfaces gives you a lot of benefits. You can always see signal levels of the equipment, even if the customers service is turned off for non-payment. You can also switch the equipment back and forth between routed and bridged without the customer needing to factory default the equipment.