Get originalTransactionId in app after completed purchase

Hi all, how do I get the originalTransactionId in an app after completed purchase. I use SubscriptionStoreView and onInAppPurchaseCompletion but I don't know how to extract the originalTransactionId. Any help would be appreciated.

From https://developer.apple.com/documentation/appstoreserverapi/originaltransactionid I read "To get the original transaction identifier from your app, use the originalID property of the Transaction object that represents the in-app purchase. " So I assume I can extract the originalTransactionId in the code following onInAppPurchaseCompletion

Regards Thomas S