Wash. Rev. Code § 6.32.030
Any person may be made a party to a supplemental proceeding by service of a like order in like manner as that required to be served upon the judgment debtor, and upon proof by affidavit or otherwise, to the satisfaction of the judge, that execution has been issued and return made thereon wholly or partially unsatisfied, and also that any person or corporation has personal property of the judgment debtor of the value of twenty-five dollars or over, or is indebted to him or her in said amount, or is holding the title to real estate for the judgment debtor, or has knowledge concerning the property interests of the judgment debtor, the judge may make an order requiring such person or corporation, or an officer thereof, to appear at a specified time and place before him or her, or a referee appointed by him or her, and answer concerning the same.
RCW 6.32.030