In order to sell your virtual goods items, each item needs to be linked to a product identifier. You can create these identifiers on their respective platforms and these will be unique.
Registration of products is done through iTunes Connect.
As an iTunes Connect user with an Admin or Technical role, sign in to iTunes Connect.
On home page of iTunes Connect, go to "My Apps"
Click on the app you want to enable IAP
Select Features tab.
Click on In-App Purchases Section.
Click on + icon.
Select Consumable or Non-Consumable product
Fill in details of the product and make a note of Product Identifier.
Save the In-App product information.
For more information, please refer Creating In-app Purchase Products.
Registration of products is done through Google Play Developer Console.
On home page, select the application you want to enable IAP.
Click on In-App products section on left.
Click on + Add New Product button. If you don't see button to create a product,
You may not have uploaded atleast one APK or
Not have setup a Google Merchant Account.
Select Managed Product and add a unique product identifier.
Click on Continue and fill in details of the product and make a note of Product Identifier.
Make sure you set the status of product to active if you wan't to use it.
For more information, please refer Administering In-app Billing.