I used one of these on my Gimson Robotics motor and below in amongst this quite long post is how I fitted it.
[EDIT…] I see the phidgets motor has a key way fitting so slightly different to mine but the same method would basically work.
https://www.modelsport.co.uk/absima-wheel-set-buggy-disc-black-1-8-2-/rc-car-products/435483