Form Submission List Builder

Hopefully someone can help with this.

I've got a Catalog Request form on my company website that takes Name, Address, E-mail Address, etc. and e-mails it as an order for a catalog to our designated e-mail address.

What we'd like to do is add a checkbox next to e-mail address to "Sign Up for our Newsletter", which would pass the data in the e-mail address field to our List Builder account.

I've already got the necessary information to pass information into the List Builder account, but I can't figure out how to work out the conditional statement. Basically, if the box is checked, the Catalog Request is submitted and then the e-mail data gets passed to our List Builder. If it's unchecked, the Catalog Request goes through with nothing else.

Anyone have any experience with this? Thanks!

What are you using for this? PHP? ASP? JS?

I didn't set this up - it looks like it's using Yahoo! Store's Pro-Forma CGI script.