averageradical

Non-technical summary: Track incoming referrer and write it to any hidden form field with the identifier “referrer” (this can be created in Contact Form 7, for example; see below). Also if the WordPress site redirects from HTTP to HTTPS, this plugin also overrides the redirect to add the referrer to the https URL.
Technical details: This plugin runs client-side JavaScript in every page in HEAD and if there isn’t a session cookie with the name “referrer”, then it writes a session cookie with the name “referrer” and the value of either document.referrer, or if that’s empty, the URL-encoded value of window.location.href. There is another client-side JavaScript in every page before which writes the value of the “referrer” cookie to any INPUT with the ID “referrer”, if it exists.
When integrating with Contact Form 7, add a hidden input field such as the following:
<pre><code>[hidden referrer id:referrer]
</code></pre>
And then add to the Mail’s Message Body:
<pre><code>Referrer: [referrer]
</code></pre>
If you’re just integrating this with HTML, just add this to any form:
<pre><code><input type="hidden" id="referrer" name="referrer" />
</code></pre>

track-incoming-referrer

Track incoming referrer and write it to any hidden form field with the identifier "referrer".

nicholas_tsim

Add useful referrer information to emails sent via any Contact Form 7 contact forms on your WordPress website. Based on the Enhanced WordPress Contact Form plugin by Yoast.
Main features are: 
* Add website referrer(s) links. 
* Pages visited on your website. 
* Search engine keywords used (if referrer was a search engine).
<h3>Notes</h3>
Can be modified to only replace [referrer] tags by uncommenting line 148 and commenting line 150. May be useful if you want to restrict which contact forms send referrer information and which don’t.

Contact Form 7 Referrer Addon Plugin

Add useful referrer information to emails sent via any Contact Form 7 contact forms on your Wordpress website. Based on the Enhanced Wordpress Contact …

track-incoming-referrer vs Contact Form 7 Referrer Addon Plugin