You can get a permit for your banquet from the Washington State Liquor Control Board here: https://www.liq.wa.gov/licensing/online-banquet-permit
What does the permit cost?
Banquet permits cost $11 per day for your event, so, a three day events costs $33. Note that you will need an email address and a credit or debit card to complete this transaction. If you do not find the email with attachment in your inbox, please check your “junk” or “spam” folders before contacting WSLCB Licensing Customer Service Desk for support.
To learn more about Banquet Permits, visit: http://www.liq.wa.gov/licensing/banquet-permits