When you have registered to book your preferred vehicle by text the VRM is not required example:
PARK 5113 3H 140 means 'Please may I park my preferred vehicle at 5113 for 3 hours? My CV2 code is 140.
To book one of your registered vehicles, send the VRM
PARK 5113 DA56FDO 3H 140 means 'Please may I park DA56FDO at 5113 for 3 hours? My CV2 code is 140.
You cannot park an unregistered vehicle by text.