You must file DC Form D-4 with your employer. Here's a link:
https://otr.cfo.dc.gov/page/withholding-tax-forms-and-publications
Note that the form on DC's own website is the 2017 form. I don't know if the 2022 form is yet available.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
