Creating a New Record, can I retrieve URL parameter?

Creating a New Record, can I retrieve URL parameter?

I have a report that is filtered using a URL variable, like this:


When I click the "add" button from the report header, I want the new record form to open and populate the Plans field in the form with the PlansID parameter in the URL. 

How do I retrieve the parameter in an On Load Deluge script (to pass into the field)?