The cities sorted by number of nonstops to Seattle per day:
Los Angeles 23 Denver 18 Chicago 14 Dallas 12 Washington, D.C. 7 Atlanta 6 Newark 6 New York 6 Boston 5 Austin 3 Nashville 2 Philadelphia 2 Raleigh, N.C. 2 Indianapolis 1 Toronto, Canada 1 Miami 1 Columbus 0 Pittsburgh 0 Montgomery County, Md. ? Northern Virginia ?
Also, Northern Virginia is the location of Dulles. Montgomery county would also use Dulles or BWI which has 3 flights per day. Interestingly, with traffic, Washington D.C. is not within 45 minutes of Dulles so Washington D.C. is probably just looking at Reagan which has 1 flight per day to Seattle.
A fairly busy airport already exists in every case, so all they have to do is provide a revenue guarantee for one daily nonstop flight to Seattle. This is very common in the industry to entice airlines to open new routes.
This requires no new infrastructure, and would only be needed prior to HQ2 being fully operational. At that point, the airline would be selling enough tickets that the revenue guarantee wouldn't be needed anymore.
I could see Southwest adding a PIT -> Seattle non-stop in a hurry if needed.