Reply-To or From: header from form input

I’m going to answer this one myself.

To show the form submitter’s email address as the “From” address in the notification email:

* On your form page, under the “Form Extra” tab, create a nickname (call it “email”) for your form input that corresponds to the user’s email address.

* Save, then click back to the “Edit” tab.

* Enter “[item email]” (or whatever you named your nickname in place of “email”) in the “From:” box under “Email-Notifications” (on the right sidebar.

Save, and you should be good to go!