On the DU online payment page, were you see "Enter the Credit Card you wish to use** " one of the drop down choices is Disney Gift Card. I put the GC number were it asks for Credit Card Number and the 6 digit security code where it asks for CVV2 number
Where it asks "Please indicate what this payment/credit card information is for" You can select add to shipboard account (or pick one of the other options)
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!