Custom Field Data from Specific Post/Page

I have added a custom field to an Authnet Specific Post/Page Form called bib_num to capture the bib number of a runner in an event. I set it up so an unregistered user can purchase a page.

I followed the tutorial Custom Pro Form Templates that Collect Custom Data to set up the fields (http://www.s2member.com/kb/pro-forms/#customizing-pro-forms), but the example of the s2-hacks.php is for a user registration page, so the add_action() method applies a filter to ws_plugin__s2member_during_configure_user_registration, and that method doesn’t seem to get triggered on a Specific Post/Page buy now type form.

How would I modify s2-hacks.php to capture the information from the Bib Number custom field?

In the form (authnet-sp-checkout-form.php). My custom field code is as follows:

<div id="s2member-pro-authnet-checkout-form-bib_num-div" class="s2member-pro-authnet-form-div s2member-pro-authnet-checkout-form-div s2member-pro-authnet-form-bib_num-div s2member-pro-authnet-checkout-form-bib_num-div">
	<label for="bib_num">Bib #: *</label><br />
	
</div>
		

How can I capture the data from Bib# and display that value on the notification email I get when a user purchases an item? The live form is at http://racevids.com/?portfolio=foot-traffic-flat-2013