Anyone have any tips for finding code-share flights on nwa.com??

Specifically, I'm trying to book an itinerary that contains NW6153 (aka CO1888) from IAH-SEA.
nwa.com tries to route me through MSP on my way to SEA (which would be great, but raises the price by about $300, at least according to ITA).

I've tried multi-city, but even when I have IAH-SEA as a city pair, and click "non-stop flights only" this doesn't come up as an option.