Worked in multiple states for same employer - 1with state income tax, 1without. W-2 only lists the taxed state and puts all the income there. How do I split that?

Basically, the Temp company covers conferences/conventions in multiple states including the two that I worked - WA and NYC.  WA doesn't have state taxes and they don't list it on the W-2 even though half the wages are from there.

Do I add it in the edit W-2 part or do I fill out a separate form to allocate that only half of these wages are NYC wages?