Employer matching contributions are not permitted to be Roth contributions. Employer matching contributions are required to be pre-tax to the traditional account in your 401(k) and therefore have no effect on your Roth 401(k) contribution basis.
If you later do an In-Plan Roth Rollover (IRR) from the traditional account to the Roth account in the 401(k) plan, the amount of the IRR becomes part of your basis in IRRs.
https://www.irs.gov/retirement-plans/retirement-plans-faqs-on-designated-roth-accounts#irr12
