By default ChannelApe will map the SKU of your variant to MPN when creating a listing in eBay. However if your SKU does not happen to be the correct MPN then you can change this by adding the “mpn” Additional Field to your variant. You can map this field to a column in your Generic Data Feed using the mapper we provide within the ChannelApe app or you can contact us to have us add it to your feed for you. After you map the MPN column and pull your products again you should see the “subtitle” field show up in the Additional Fields section of your variants.
Once you push your products to eBay you should see the MPN populated on your listings.