Wash. Rev. Code § 50.12.060
The commissioner is hereby authorized to enter into arrangements with the appropriate agencies of other states, foreign governments, or the federal government whereby services performed by an individual for a single employing unit for which services are customarily performed in more than one state shall be deemed to be services performed entirely within any one of the states (1) in which any part of such individual's service is performed, or (2) in which such individual has his or her residence, or (3) in which the employing unit maintains a place of business: PROVIDED, That there is in effect, as to such services, an election by the employing unit with the acquiescence of such individual, approved by the agency charged with the administration of such state's unemployment compensation law, pursuant to which all the services performed by such individual for such employing unit are deemed to be performed entirely within such state.
RCW 50.12.060