most likely if you purchased it from the link of Norton then you'll receive a confirmation email about the order they do provide PK on that link.
if you bought it in a local store then Product Key will be inside the box or a card associated with it.
Or if you are not really sure just ask help from the chat support of Norton. www.norton.com/chat
If it was a purchase direct from Norton it'd also be listed in your Norton account. You can login and check by going to www.mynortonaccount.com. Generally any Norton purchase requires you to create one of these accounts.
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