Campaign Monitor form signup to additional email address?

Well it somewhat depends on what the user’s expected experience is at the remote site - if the receiving script simply bounces the user back to your site after processing the form, you can safely send the data from your script to theirs via [FPHP]cURL[/FPHP]. If the user is expected to interact with the remote site, however, you’ll probably be better off intercepting the data at submit-time with javascript, firing the data at your local code, and then allowing the form to process as normal.