Salesforce form pass GA data - google-analytics

Question

I am using Salesforce's Web-to-lead feature. It basically creates a form that inputs the data in a new lead for every submission. I'm wondering how to dynamically populate the hidden fields in the code below (Refer Url, Landing Page URL, GA source / medium / campaign)
I tried the code suggested on this blog post, unsuccessfully.
This is what the html for the form loks like:
<form action="https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8" method="POST">
<input type=hidden name="oid" value="XXXXXXXXXXXXXXX">
<input type=hidden name="retURL" value="/thankyou.html">
<label for="email">Email</label><input id="email" maxlength="80" name="email" size="20" type="text" /><br>
<label for="first_name">First Name</label><input id="first_name" maxlength="40" name="first_name" size="20" type="text" /><br>
<label for="last_name">Last Name</label><input id="last_name" maxlength="80" name="last_name" size="20" type="text" /><br>
<label for="phone">Phone</label><input id="phone" maxlength="40" name="phone" size="20" type="text" /><br>
<label for="company">Company</label><input id="company" maxlength="40" name="company" size="20" type="text" /><br>
<label for="title">Title</label><input id="title" maxlength="40" name="title" size="20" type="text" /><br>
<div style="display:none;">
<label for="lead_source">Lead Source</label><select id="lead_source" name="lead_source">
<option value="Quotefx Web Inquiry" selected>Web Inquiry</option>
</select><br>
Refer URL:<input id="XXXXXXXXXXXXXX" maxlength="255" name="XXXXXXXXXXXXXX" size="20" type="text" /><br>
Landing Pg URL:<input id="XXXXXXXXXXXXXX" maxlength="255" name="XXXXXXXXXXXXXX" size="20" type="text" /><br>
GA Medium:<input id="XXXXXXXXXXXXXX" maxlength="255" name="XXXXXXXXXXXXXX" size="20" type="text" /><br>
GA Source:<input id="XXXXXXXXXXXXXX" maxlength="255" name="XXXXXXXXXXXXXX" size="20" type="text" /><br>
GA Campaign:<input id="XXXXXXXXXXXXXX" maxlength="255" name="XXXXXXXXXXXXXX" size="20" type="text" /><br>
</div>
<input type="submit" name="submit">
</form>`