You don't need a special code. Your account number works as a gift card. So, if you are online, there are two options:
1) For some purchases, DVA will actually be a payment option -- enter your account number;
2) If no DVA payment option appears, just enter your account number in the space for gift card
3) On the phone, the CM can also use your account number as a gift card. Some CM's don't know that and you may have to tell them. It seems like more are becoming familiar.
I am using DVA solely as a way to consolidate Disney GC's that I buy at Target with my Redcard, thereby saving 5%. Initially, I was having trouble making larger purchases with my stack of $50 gift cards, because they only want to take two forms of payment online and on the phone, in some cases (Target only carries $50 increments so you have to use a lot of cards to make larger purchases). I have since seen info on this board of the procedure that you may have to educate CM's on, in order to use more than two forms of payment. Still, I would rather use DVA (assuming it works well) to enter the cards at my leisure. I entered 30 cards last night at home, while watching The Little Mermaid on TV (double Disney tasking!).
I leave for DL in one week, and am using it as a test case, because I did have some lingering concerns about DVA and want to make sure it is widely accepted before purchasing and loading enough cards for our much longer/more expensive WDW trip in November.
I plan to charge everything to the room and pay off using DVA. There is no advance payment option on Room Only Reservations. They will also not take GC as payment for your first night stay requirement to book. The only option is to ask the front desk in person to credit back the credit card you use to book and then reapply the charge to GC. Again, I am hoping for both my sake and the CM at the front desk that "one and done" works with DVA account number and we aren't standing at the front desk entering individual GC, especially since I will likely have to stop by several times to pay on the account to avoid them auto charging credit card on file. Again, if that happens, they can reverse the charges on your credit card and reapply GC/DVA.
It seems like a lot of work, but if it performs as advertised, it saves a significant amount of money and lets me enter GC's on my time. I will report back after next week!