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I have a iOS mobile application where I am using OFFER CODES year by year where the users can login & re-new the subscription for the application. I have finished my development in my Backend & my iOS Frontend side & I have tested both the platforms individually and they are in the working condition. I need a way to test this after the integration using test flight with a verified application or using sandbox environment using my local application.

I intend to use Apple Store Connect API for making & updating OFFER CODES in my subscription. I have created the GET requests needed using an API Key I have generated but the POST requests are not going through and getting an error in the process. https://api.appstoreconnect.apple.com/v1/subscriptionOfferCodes This is the API that I am trying to hit using the POST request but the process is not going through for some reason.