Orders tracking of external websites

Hi all,
I have a problem, I have a site that passes through links to external users, now I got permission from these affiliated sites to place an order tracking code on their site in the thank you page, this is the problem, or if I give him the pwik code on their thank you page Verdo all tracings of all orders, not just those from my site. So my question is this:
how do I plot only the orders of users coming from my site on third party sites?

I hope I was clear enough.