Well, I have done this in the past and maybe the things are the same now.
The approach I used is the following.
1) prepare a script name addlead.php using the getresponse API. I call it with three parameters
example: addlead.php?name=<customer_name>&campaign_id=<camp aign_id>&email=<customer_email>
With this script I managed to add lead to every campaing I needed.
2) use the Clickbank's Instant Notification Service (e.g. clickbankIPN.php ) where you can "understand" what product you have sold and the call the addlead.php script to add the customers to your campaign.