In order to activate Fair Choice in your iOS app, you need to create either a subscription or an in-app purchase and link it's ID to the corresponding Asset in the consent settings page.
This creation can be managed on App Store Connect for both subscription and in-app purchase. To create a subscription, you can refer to Apple's documentation for subscription creation. To create an in-app purchase, you can refer to Apple's documentation for in-app purchase creation.
When you have successfully created your in-app purchase, you can copy & paste the ID of your newly created in-app purchase on the consent settings page.
The ID can be found on the following location in the administration interface: