You must select an airline before using this benefit. Please read the Wikipost at the top of this thread, and follow the link to the selection page on americanexpress.com
After doing a complete PTF I called Amex and asked if they would reimburse me for the charges since no one at Amex told me I had to enroll first and to my surprise the agent said she'd give me half back or $100 credit (which I thought meant using $100/$200) but she then added I would still have the $200 to use after selecting my airline, so essentially giving me $100. Pretty generous I thought and highly YMMV.