My client is using an external email newsletter service. I've been able to set-up the newsletter subscribe page fine. No problems.

My question is regarding customers that create an account when purchasing. Is there a way to pass their email address over to the external service when they select "subscribe to the newsletter" at the bottom of the create account form?

Right now, I currently have this disabled -- but would really like to have this form send the email address to the external script/service.

Any ideas? Thank you for your help!