For fully automated installs, preseed/url should itself be preseeded (via kernel command line, DHCP, or syslinux.cfg on customised media).
I yisbedden ummiden iurmanen, tawila tuzwirt /url ilaq ad tili d tawila tuzwirt (s uzirig n tiludna n kernel, DHCP, neɣ syslinux.cfg ɣef umidya udmawan).