Cookie dropping script

We own two URLs (Site A and Site B) what we want is the ability that when people visit site A then without any user actions we want the ability to activate a javascript code on site B. This code will open up a 3rd party website that will drop a tracking cookie. The referee left on the users visit after the cookie is dropped will be for a page on site A. For a full explanation on how this will work please contact me and I will be happy to send you a specification document.