How to offer 10% discount to new email subscribers?

[P_F21]

Hi, 

I'm currently using the Dawn theme and I have a Mailchimp integration. I'd like to automatically offer all new email subscribers 10% off upon sign up with a pop up image. Can someone point me in the right direction please? It would be great if I could have different codes generated for each sign up that expire.

Thanks in advance!

Perri