Have a look at the terms and conditions that they are legally required to display. There may be a number you can call if it is not clear
It is probably free for motorbikes as long as you park in their designated motorbike place and do not occupy a car bay. However each park may have a different policy so don't assume anything.
The rules for this differ in nearly every car park you go to, not many cater for bikes.
There are designated bays for bikes but they are few of them, if you park in any car bays they can simply give you a ticket for not displaying even though your displayed ticket can easily dissapear.