Hi Bianca Longhi,
it is possible to enable WNS for an application in Microsoft Partner Center without submitting it for review. You can access the necessary credentials (Package SID, Application ID, and Client Secret) for testing purposes once the app is registered in Partner Center. Submission is only required if you plan to publish the app to the Microsoft Store.
Please follow below document, which i believe will definitely help you on the requirement
If you have any further assistant, do let me know.
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