Hey all,
I've also been having issues with this but think I've found a good work around.
You can eliminate the need for a confirmation email by adding google reCAPTCHA to form blocks and promotional popups. 'How to' info here - https://support.squarespace.com/hc/en-us/articles/115013193528
Then you can change the post-submit message to display the discount code - here's how to do this https://support.squarespace.com/hc/en-us/articles/206566737
Hope it helps :)