i will use html form
and
code on pageload events:
if (Request.QueryString["tx"] != null && Convert.ToString(Request.QueryString["tx"]) != "" && Request.QueryString["st"] != null && Convert.ToString(Request.QueryString["st"]) != "")
and on page use this
<input type="hidden" name="return" value="http://xyz.com/thankyou.aspx" />