Silly question around these travel credits and 15% off coupons (USTA ones in my case).

Can I do this online? Or do I do it over the phone and mail things in? if i do mail things in, do people do registered mail?

Or if I am going to be at the airport later, can I just make reservation now/ticket now and turn in stuff at the airport in a week?

ORD is my airport.

TIA