Although this method works, be careful as they will not be water resistant with the tips pointing up like this so best not to use this method if you cycle in the rain.
He also used a more permanent means to attach the button as he describes:
You will note that the Velcro fitting supplied with the horn button is far too large for the narrow bars on our trikes and so we have used two narrow black cable ties. The horn button is easily pressed with the side of the hand whilst braking. They do work very well!