You must register a trade name, or “doing business as” name, per RCW 19.80.10 for:
Registering your trade name does not protect the name from use by others. A business can register an unlimited number of trade names.
A trade name will remain registered indefinitely until the owner requests that it be canceled. There is no charge for canceling a trade name.
If you also want to file a trademark or service mark, you may want to consult an attorney. If your business activities will extend beyond Washington State, it may be to your benefit to file for a trademark with the U.S. Patent and Trademark Office. If your business activities will stay within Washington, it may be to your benefit to file for a trademark with the Secretary of State.
See also the Trade Name Registration- Business Bulletin
Add a state endorsement to your business
Add a city license to your business
Apply for a new business license
Setting up your business licensing and tax account
How to add a city license endorsement
Complete the following and mail to:
Business Licensing Service
P.O. Box 9034
Olympia, WA 98507-9034