Paste the URL below in the Settings → Notifications → Form User Options → Redirect to a web page box and make the indicated adjustments.
a) http://www.yoursite.com/file.php – Replace this with your custom page.
b) yourrefid – Here, you can use any variable you want (ABC, R, anything). The variable can be different from the one you have put in the Reference ID box, in Notifications.
c) =requestRefID – This part should be left exactly as it is.