I don't think there's any "command" in MS PPTP to do it;
However, I found this which may allow you to do it once the initial PPTP link is up:
Let us assume the gateway router is an ISP Router. When the PPTP tunnel comes up on the PC, the PPTP route is installed with a higher metric than the previous default, so we lose Internet connectivity. To remedy this, modify the Microsoft routing to delete the default and reinstall the default route (this requires knowing the IP address the PPTP client has been assigned; for the current example, this was 172.16.10.1):
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...