title: How can I retrieve the values of controls in the form that posted?
body: <p>I know this has got to be the simplest-sounding question ever asked about ASP.Net but I'm baffled.  I have a form wherein my visitor will enter name, address, etc.  Then I am POSTing that form via the PostBackUrl property of my Submit button to another page, where the fields are supposed to be all re-formed into new hidden fields, then POSTed again to Paypal.</p>
  
  <p>My problem is I cannot get at the values entered by the visitor in the original page.  Any time I put in "runat='server'", ASP.Net completely changes the ID of the control, making it impossible to figure out how to access.  In the POSTed form I tried <code>Request.Form["_txtFirstName"]</code> and that turned up null.  Then I tried <code>((TextBox)PreviousPage.FindControl("_txtFirstName")).Text</code> and that was null, too.  I've tried variations on those.  I cannot figure out how I'm supposed to get at these controls.  Why does this stuff need to be so difficult?</p>