What I need now is for these same links to also populate a hidden HTML form field so that if a visitor follows this link to the site and then uses the Contact Form, a hidden field sends "Referred by Adwords" to the recipient.
So if visitor follows link [[login to view URL]] and then sends a message through the HTML form, a hidden form field will transmit "Referred by AdWords" in the message. There will be several different referral sources and one default for when a tracking link is not used.
[[login to view URL]] | Google
[[login to view URL]] | Microsoft
[[login to view URL]] | Whatever
Any method is fine so long as it is simple enough to use and reliable.