You only contact the *A carrier that you have your miles on (let's say UA).

UA has two types of awards: UA (or MP) awards flown on UA metal (which can be "saver" or "standard), and *A awards flown on partners' metal (which are saver-level) and are un-changeable once travel has commenced.

If there is no "UA award" availability, you can try for a "*A award." It's that simple.