Reply to is already included as part of the CSS Form Builder wizard.
to use reply to when first createing the form:
On the form Function tab, click the Edit button next to Send email Settings to edit the Universal Email settings.
If the form is already created, go to the server behaviors list and double click the Universal Email behavior to edit it's settings.
In the Universal Email wizard, click the From Button, then set the Reply To address in the Additional Headers section.