Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hi Gaveshi,
If you're using a Microsoft 365 subscription, you should not need a product key after the subscription has been activated. The apps normally activate by signing in with the Microsoft account that owns the subscription.
Since you are being prompted for a product key and are unsure which email address was originally used to purchase the subscription, the first step is to identify the Microsoft account that owns the license. Try any email addresses you may have used in the past and check if the subscription appears under Services & subscriptions when signed in.
Also since you mentioned that this is a subscription, it would also have been billed on a recurring basis, either monthly or annually. Therefore, the account that received the renewal receipts or from which the subscription payments were charged is often the account that holds the license.