Hi @Seller_vDHc0RrFfgEpY, have you already followed the instructions in the "List a customizable product" section of this help page?

"To list a customizable product, follow these steps:

1. Create your listing first, as if it was not customizable, but ensure that you enter "0" in the Quantity field on the Offer tab. If you set the quantity as zero, it ensures that nobody is able to buy your product without customization.
2. Go to Inventory and select Manage all inventory.
3. Search for the listing that you want to configure, click the down arrow next to Edit, and click Add/edit customization information.
4. Add the customization details for your product. Ensure that you upload a preview image and include a zero-cost customization option. This is only needed for listings where $0 customization is not present.
5. Update Quantity of your inventory to at least "1" once your product has been enabled with customization. This will make your product buyable."

You may also want to ready this help page, "List customized products with Amazon Custom."

-Blake