Look up lotteries on your regulator's website. If an entrant is required to give consideration (buying a ticket, being a customer/having an account, etc.) in exchange for an opportunity to win a prize worth more than the consideration they provided, you have a lottery, which is prohibited.
That being said, there are ways to conduct a drawing that keep you in the clear, such as making it open to the general public and not providing your customers a greater chance of winning.
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