In my experience, you have to use the Disney visa for the deposit, but you can use other credit, debit, or Disney gift cards, or your Disney reward dollars for the balance. There will be a discount code in the announcement. If you have something booked that fits the discount qualifications you can call and give them the code to have it applied if there is availability. I haven’t done this, but I heard last year that if you booked online and have MDE set up, there should be a modify button that you can select to be able to apply the code to. It will bring up the rooms that qualify for the dates you have booked and you can change your reservation to a discounted room. The danger of everyone booking a room in advance in hopes of getting a discount is that if enough rooms are already booked, Disney won’t feel the need to discount any. They only discount when there is a lack of reservered rooms to help increase bookings. Or so I’ve read. The advice is to book ahead only if you are going with or without a discount. Good luck!
