When you are creating a suscription form, you may want to add a custom page, for instance to thank your new subscriber, or to offer an ebook to download.
Mailrelay has an opt-in form generator, which allows you to create your forms with custom pages, you can find all the necessary information here:
http://blog.mailrelay.com/en/2014/11/14/improvements-in-the-mailrelay-autoresponder-custom-form-generator