Your customer number is printed at the top of your business license tax certificate.
You can also lookup your customer number/business number here: https://www.seattle.gov/licenses/find-a-business.
Additionally, the City of Seattle customer number can also be found on the taxicab/for hire medallion certificate.
If you do not have a City of Seattle business license, you can submit an application online, in person, or by mail. For details and application help please visit https://www.seattle.gov/licenses/get-a-business-license.