See the attribute output for the shortcode, and set it to "url": WP Admin > s2Member > PayPal Buttons > Shortcode Attributes > output
I don't know if that field in the theme will parse a shortcode, though.
If not, first parse it in a page while logged out of your account, then copy the URL from there and paste it in the field. The problem with this is that the URL will always be for new signups, never linked to an existing account to upgrade, because it's not being parsed for each separate existing user referencing his account.
I hope that helps.