<!-- http://www.domainname.com?utm_source...ad&utm_medium= -->
<form action="http://www.domainname.com/" method=GET>
<input type="hidden" name="utm_source" value="- put value here -">
<input type="text" name="utm_medium" value="your email address">
<input type="submit" value="Submit">
Note: Include the utm_source, and any other values that need to be passed, in hidden input fields, except for the utm_medium, which is user input.