How do you normally accept leads from iContact? Do they provide a formbuilder of some kind? If so, you should be able to look at the source code to see the form action url -- use that and any other "important" fields they provide when mapping, very similar to the Salesforce example (http://wordpress.org/extend/plugins/forms-3rdparty-integration/screenshots/).
Do they provide an API? Because that would work as well -- if it accepts HTTP POST requests you can use this plugin as-is, otherwise you'd have to write a hook to intercept the submission step, and this plugin isn't really set up to allow you to do a different kind of submission. I think I know where to put a hook that would allow you to make a different submission if you needed it, but I'm planning to sunset support on this plugin in favor of its upgrade: http://wordpress.org/extend/plugins/forms-3rdparty-integration/, so any improvements would show up there.