Envato Sites is a simple to use website builder, so we've shied away from custom coding which can be complex. It's not possible to edit HTML on your website.
For a mailing list subscription form you can add a MailChimp subscriber popup.
To verify website ownership with Google you can add your Google Analytics tracking code.
If you'd like to add custom HTML to your website for a purpose not mentioned above, please get in contact with our support team. We'd love to hear your feature request and we may have another way of helping you set up what you need!