Some things are far more simple than you expect.  And far less documented than they should be.

All I want to know is what URL will the page post back to when a submit is fired.  It’s a simple thing.  It shouldn’t take me 45 minutes to find it in Google.  Well it did.

Finally, I found my answer.

Open the page source for the page in question. Find the <form> tag.

<form name="aspnetForm" method="post" action="/Checkout/Payment.aspx" onsubmit="javascript:return WebForm_OnSubmit();" id="aspnetForm">

The action attribute contains the URL where the form data will be posted to when you click on the button in the web page.