The email that customers recieve has the Discount Coupon code on it ...

When they come to the site via the URL in the email ... it is automatically added when they login ...

If you want to automate this on completion of signup ... you need to only add 1 line of code to the /includes/modules/create_account.php

This will remain valid until the user logs out or the session times out ...

If either should happen, they can then apply the code during checkout ...