You need to obtain the wiring diagram for the relay for this horn used on the 12GS. I've had mine on there for several years and it works perfectly well without being shut down by the Canbus.
The wiring however, is not how you'd normally wire a horn using a relay. I don't have the schematic handy, so I can't tell you how to wire it so it works, but there is a way of doing it. I do recall there are 2 leads to the battery, 1 to the bike's +ve horn harness..