It can easily be done online...
First book your Ding rate until you get to the part where you enter payment.
Open a NEW window and go to the Travel Center and cancel your original flight. You will be given a confirmation number for ticketless travel funds.
Now reopen the Ding window and select View/Apply Ticketless travel funds to pay for this fare. It will deduct your fare from the funds.
I've done this a couple of times, just did it today and saved $71!
HTH
This is a quote from laurabelle and sounds like the best way to do it.