Asp.net Paypal Return Url Requirement

Question

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" />