Originally Posted by jhayes_1780
Welecome to FT... Unfortunately, you cannot book with or pay UA for the flight. You have to book with AC.
Not necessarily true. If you have an MCO (Misc. Change Order) you can use it for UA flights as long as they have an AC flight number (codeshare).
See:
http://www.flyertalk.com/forum/showthread.php?t=530910