Add Prefab to AR object menu

It's easy to add your prefab into AR asset bar

Select Canvas and you can see Object Handler component at the bottom of Inspector.

Expand AR Objects. Set Number of objects you want to display at the top end of the Object Handler compenent. Click + button to add new item into list. Set name as "Car" (Your prefab name). Drag and drop newly created Prefab in to Placement Object. If you have icon for object convert it to sprite and drag and drop object Icon place.