To make Amazon labelling the FNSKUs for you, EAN codes have to be displayed on your products. This requires that you have stored an EAN code for your product in the listing, so that Amazon can clearly assign the product.
This is how you can use the service via VentoryOne:
1) Click on the hamburger menu of your respective SKU to enter the SKU settings:
2) Scroll down to the settings: "Miscellaneous".
Change from 'Seller' to 'Amazon' and safe.