Hi,
We don't allow for that in order to avoid spam. If you add that, it means than anyone can have your website send an email to any email address they want. That's a good way to have your website email server flagged as a spammer and have all your website emails end up in the spam by default or even get rejected altogether by some email servers with stricter rules.
I don't think that the benefits of having the contact email CCed is greater than all the other emails ending up as flagged as spam so I don't recommend doing that.
Now, if you really want to do it, you can add such code in the preload of the contact email via the menu System>Emails:
Code:
$mail->cc_email = $data->element->email;
