In order to add fitment attributes to an Amazon item, you need to use the following attributes when creating the Amazon listing: Part type ID, Vehicle Fitment Code, and Vehicle Fitment Standard. You can find these attributes for corresponding categories in the Amazon configurator.
These attributes can also be filled in automatically with the information from item Extended Properties. Please refer to this documentation article for more information on setting up attributes and linking them to specific extended properties. Full details on specifically Amazon configurators are available here.