You can't add this service proactively, as you can with the plans shown above; you apparently need to run out of payments in a month and try to schedule an additional one in order to see this pop-up dialog box:

When you get that dialog, and click enable, you authorize Quicken to automatically add the additional payments for $3.00 in any month in which you run out of payments. (In months where you do not exceed your base level, you won't be charged.)