I had this problem back in April. Before I submitted the form I called the plat line, who referred me to nwa.com support, who said to fill in the form and the refund would be processed no problem. Fortunately you get an acknowledgement email right after the form is submitted. Two or three weeks later, when the refund hadn't appeared and the reservation still showed active, I called. Turned out the request was received but denied for no apparent reason (I had submitted it well within 24 hours of purchase.) Then they didn't want to refund it because it was now too late for a full refund. Eventually a fax to the refunds department with copies of all of the emails and other documentation got the refund issued.

I'm not sure there is a lesson in any of this, because if you get the message and the form the OP refers to you are stuck. One thing I did not try was simply calling in and requesting the refund by phone. They may be able to do that and just bypass the web site entirely. However, with a phone call you don't get a printed record that could be useful if things don't work.