There are different ways you can do this.
In writing: Write the Office of Administrative Hearings (OAH), P.O. Box 42488,
Olympia, WA 98504.
Verbally: You can call OAH at (360) 407-2700 or 1-800-583-8271 or tell DSHS that you want a hearing. DSHS may have you follow up with a written request.
When will the hearing take place?
If it is an emergency, you should call OAH to ask them to hold the hearing as soon as possible. This is called anexpedited hearing. Otherwise, your hearing will probably be 20 days or more after you ask for it.