Are you actually redirecting the user to the third party site? Does the user need to enter data on the third party site? (It sounds almost like you're talking about PayPal integration.)
I think that you'll have to be more specific to get any good answers.
That is correct.
We take the user to the paypal like site where the form is pre-populated with data gathered from the seam page. There is more data that is added once the page is redirected to 3rd party appl. Once the form is submitted, the transactionID needs to be sent back to seam application. This data gets lost when the page gets redirected back to seam.
Shall post the code snippet shortly.
Do you append jsessionid on the return url?
Make use of java servlets for redirection i mean write one servlet to post data to paypal and another for receiving data, Once you receive the response redirect it.