<?php echo S2MEMBER_PAYPAL_RETURN_URL; ?>
This will output something like: http://www.example.com/?s2member_paypal_return=1
(this is used as the Signup Confirmation Page, and also handles PDT/Auto-Return)

---- s2member Shortcode Equivalent ----

[s2Get constant="S2MEMBER_PAYPAL_RETURN_URL" /]