Auto populate with $_GET and submit to 3rd party

Hello,

I auto populated a hidden field with some $_GET variable (with url ?source=something) using this plugin

I can confirm that it echoes to the html page by looking at the source.

Now i can’t map it to a 3rd party field.
How would you suggest me to auto populate a hidden field with some $_GET variable, then submit it to the 3rd party URL?

Thanks!

http://wordpress.org/extend/plugins/contact-form-7-3rd-party-integration/