1) Make sure you already have a form on your website. If there isn't one, you can add a form by clicking and dragging it from the main toolbar.
Don't know how to add a form? click on : How to add a form to your website?
2) Select the form that you want to add the Email address to
3) Enter the desired Email address into the Email address field in the form's settings menu
Make sure you have added a submit button to your form so that the completed forms can be sent to the Email that you have defined
4) All changes to your website will take effect only after publishing your website.
5) We also recommend saving your website after making changes.