FWIW, ANA seats are usually selectable when booking on UA (at least for paid fares) but only if there are free seats available.

With miles (which I only do for domestic flights so things may be different), you just need ANA's confirmation number (which UA provides) and your name exactly how it appears on UA except with no spaces (e.g, if you have a middle initial on UA, you need to include it: Maryl not Mary or Mary L).

If you have status with UA be sure to reenter your Mileage Plus number at check in as it often seems to disappear, making it trickier to get Star Alliance perks.