Buy a cable lock at a motorcycle shop or from an online retailer. The Bully integrated cable lock is a cost-effective option. Ensure that the cable is at least a little longer than your arm and coated in plastic so it does not damage your motorcycle or jacket.
Uncoil the lock, then thread it through the arm hole of your jacket. If it won't hurt your jacket, bunch up the material to make for a smaller package.
Identify a section of the frame or a section of your front tire that you can loop the cable around. Do not damage your spokes or brake components. Take care not to touch the engine or exhaust; these surfaces may be hot and burn you or damage your equipment.
Use a bungee cord to wrap your jacket in a small bundle, then tuck it under your motorcycle. If you are concerned about damage from the sun, weather or pavement, put your jacket inside a bag for additional protection.