Simple Backbutton Javascript Task

I've been running a simple backbutton script on my php landing page, which uses the php GET command to pull a url parameter from the url and add it to the script url. I'm now using a landing page builder (Convertri) where php is not supported. So I need this script to accomplish the same task with only Javascript so that it is compatible with landing page builders. Should be an easy task for someone who knows what they're doing so I'm looking to get it done today if possible. I've attached a screenshot of the script. Thanks, Scott