There is already this feedback / feature request: https://feedback.shopware.com/forums/942607-shopware-6-product-feedback-ideas/suggestions/50220672-add-cc-bcc-and-reply-to-features-to-the-mail-func.

In addition it should be possible to define an email adress, that custom forms (or default forms) can reply to.

Of course one can add an email field in the form fields. But this cannot be used to open the form mail and click on reply to "customer email"