Contact Source. How to pass source or tag variable in URL?

It would be really helpful to know how somebody got to our website! We are storing a cookie to indicate which ad, or which person led the contact to us. Now I want to pass that field to DT in the URL so that as the contact submits their form, the dispatcher will know where they came from (possibly a city, a team, or a person).
If someone can at least point me to the section of code, or the variables to set, that would even help. I would like to create a snippet to store this info.

Another approach would be to do the form on the host website and just send the fields to DT somehow.