Did you include an email when you went through the purchasing process? You also receive an email after that with a link to the latest download of the game. In that email, below the "Links to Our Apartment" section of the email, you will see a "Your key for customer support reference:" in small but able to be seen print. That digits that follow : is your identifier as a customer.
If you did not supply an email during the purchasing process, I would recommend going onto your banking app and looking for the merchant and transaction IDs. Those can be used to find you on their end of things.