when I fill it into the form builder, it doesn't show in the code.
Once you've added a custom email address in the "Email Notifications" tab, you'll need to click on "Save and go back to the form builder", and then on " Add this form to my post" to update the contact form shortcode with the new parameters.
Once you've done so, the shortcode should include your email address in the [contact-form] tag, like so:
[contact-form to='ab@cd.com'][contact-field....
Let me know if it helps.
@doncobb I replied to your other post here.