US-Codeshare M fare, Y-Up?

I found one fare from IAH-SFO-HKG in US codeshare that gives you M fare. I am wondering if you are going to be able to get instant Y-up if you put in the UA-1K number in the reservation after the ticket is confirmed. To my understanding, codeshare just doesn't matter, but it's a wonderful fare M because UA just gave V for the same route at the same price.
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