This subject was brought up on another thread a few days ago so I contacted the State Patrol. This response came today.
Hello Ma’am, thank you for contacting the Washington State Patrol. The answer to your question is found in Revised Code of Washington (RCW) section 46.37.530 subsection (c). Based on the guidelines set forth in this subsection, an operator and passenger would not be required to wear a helmet while operating this motorcycle (defined in RCW 46.04.330 (2)). This is a result of the Slingshot including a steering wheel, seat belts, and a partially enclosed seating area. Depending on the model of the Slingshot, the use of goggles, or a face shield may be required if there is not a windshield present (RCW 46.37.530(b)). We appreciate your question, have a great rest of your day!