You need to take a look at the hooks of the Contact Form 7 plugin website.
Browse the documentation for the actions you need.
Have a look at this topic: how-to-hook-into-contact-form-7-before-send
All you need to do is to add your link with the hash into the e-mail.
In addition, also have a look at “wp_mail()“.
That is the mechanism which triggers an e-mail.
So, catch the e-mail before it is being sent, add your link to it, then trigger the actual mail.
I know, this isn’t a complete answer, but adding your own work to it should get you in the right direction.