Per e-mail I just got:
Use code STREAMMORE under Account -> Subscription & Billing -> Enter Code
Extended my clearly visible Next billing date one month further than it was.
Edit: "*Enter promo code and get one month free of your CBS All Access Limited Commercials or Commercial Free plan. Offer expires at 11:59 PM ET on 9/4/2020. At the end of your free month, your full price (plus any applicable taxes) subscription will resume and your card automatically charged on an ongoing basis, unless you cancel via your account prior to the end of the free month."
Edit 2: After applying the code the field to enter any additional codes does not show up anymore for me, so if you are on a previously used code you might not be able to add this one, for those trying to stack.
Edit 3: As warned by those downthread, keep track of your billing and associated dates of charges to double check them especially if using Paypal.