We can force it, by using Regedt32 to navigate to:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\PPP\CHAP

On the Edit menu, Add Value name OfferMSCHAP, a type REG_DWORD, and enter a 1 in the DWORD Editor.

Once this value is set, all incoming PPP connections will be offered MS-CHAP. MS-CHAP will be used to authenticate each PPP user against the Radius server that your RAS server uses.