Registration must be done in person at county election offices.
People in King County can register at the county's Election Annex on Fourth Avenue or at the King County Elections headquarters in Renton.
Downtown Seattle: https://www.kingcounty.gov/depts/elections/about-us/contact-us/directions.aspx#annex
Renton: https://www.kingcounty.gov/depts/elections/about-us/contact-us/directions.aspx
Registration in Snohomish County is at the elections office in Everett.
https://snohomishcountywa.gov/224/Elections-Voter-Registration
Pierce County residents can register at the county election center in Tacoma and the Kitsap County site is at the elections site in Port Orchard.
Tacoma: https://www.co.pierce.wa.us/328/Elections
Kitsap: https://www.kitsapgov.com/auditor/Pages/elections.aspx