No need to display it on a motorcycle. Carry it with you.
http://www.discoverpass.wa.gov/faq/
"Q. Where should I display the Discover Pass?
A. The Discover Pass must be visible in the front windshield of motor vehicles licensed for street use. For vehicles without a windshield, the pass must be displayed in a prominent location on the vehicle. Motorcycle riders may carry the pass with them. "