Found it: “You first have to add a ‘Text’ field to your form, give it a label, and set the validation to ‘E-Mail’. Now save the form. Then, under ‘E-Mail Notifications’ (on the right hand side of the page), select the form item you just created under ‘Send to (user entry)’. Only text fields will show up in this list, and only saved fields as well. If the field does not show up, make sure you have saved the form, then check again. Although you can choose any text field, it is highly recommended that you set the validation for the field to ‘E-Mail’.”