SKStoreProductParameterProductIdentifier
The key representing the product identifier for the promoted product you want the store to display at the top of the page.
Declaration
let SKStoreProductParameterProductIdentifier: StringDiscussion
The value for this key is an instance of NSString.
When your app uses an SKStoreProductViewController to render an app page for another app, you can optionally choose to highlight an in-app purchase by displaying it at the top of the store page. Set SKStoreProductParameterProductIdentifier to the identifier of the product you want displayed at the top of the page.
The product indicated by the identifier must be set up as a promoted product in the App Store, otherwise the identifier is ignored. See Promoting In-App Purchases.