Redirect??

How do I modify this code to direct to pop up window and say thank you. Mail Sent.  I tried to fill between"", but no luck.

'*******************************************************
'Redirect to thank you page or write thank you
'*******************************************************
IF form_redirect = "" THEN
Response.Write("Thank You. Mail Sent.")
ELSE
Response.Redirect(form_redirect)
END IF

I created a thankyou.asp then change the code to thankyou.asp.  It still jumps to an empty page says Thank you. Mail Sent.

IF form_redirect = "" THEN
Response.Write("Thank You. Mail Sent.")
ELSE
Response.Redirect("thankyou.asp")
END IF