dhpc over pptp

A remote site only has user workstations. They are on DHCP to obtain their IP address. They must contact the DHCP server over a PPTP link. There is no default gateway configured in DHCP or the PC itself.

How do the PCs in the remote site find the DHCP server over a PPTP link? How does the remote site know to pass things to the router with no default gateway configured?