url variable

I want to determine, from what domain the client comes, that makes an order. That means f.e.:
a banner is placed on domain.de, the link in this banner shall create a cookie-variable on http://www.wie-gemalt.de, so the link is http://www.wie-gemalt.de?partner=domain

in order-form I want to deliver this variable “domain” my email, so that I can see, that the order had it’s origin by this domain

The page I need help with: [log in to see the link]