Tommaselli Quick Action Throttle

I have recently purchased a Tommaselli Quick Action throttle and the POLINI gas line from sip for my vespa.
I know how to connect the throttle to the Gas Line (this is a piece of pipe that goes through the head stock in place of the original throttle).
BUT, does anybody know how to fix the the pipe into the headset top so that it does not turn when you twist the throttle?. SIP supply no instructions and theres no info in the parts catalogue.

Regards
Peter