Finding originalTransactionId to extend subscription renewal date

I am using Revenue Cat to set up in app purchases in my React Native app. I want to build an API that extends the susbcription renewal date using the App Store Server API for my iOS users. This is the PUT API call:

https://api.storekit.itunes.apple.com/inApps/v1/subscriptions/extend/{originalTransactionId}

The path parameter includes the originalTransactionId (required, the original transaction identifier of the subscription receiving a renewal date extension). However, I am unable to find the originalTransactionId on Revenue Cat.

Here is the Apple documentation: https://developer.apple.com/documentation/appstoreserverapi/extend_a_subscription_renewal_date

How do I find the originalTransactionId?