Originally Posted by
HereAndThereSC
As for the original question...
US reservations on UA codeshares generate a UA reservation number. IF you have it, you can modify/assign seats via that.
Other than that, you should be able to find the reservation via UA's website by using name, date, etc, and then you can assign seats. Don't expect too much as far as seats though.
HTSC
Actually if you are booked on a UA flight with a US flight number, the UA website will think its another airline and wont let you do anything with seats in advance. you have to call in with regards to seats.