how do I include a URL parameter as a hidden form field?

Hi,

I am not exactly sure what it is you are trying to accomplish.  Here is our guide on how to pre-populate fields:

http://www.jotform.com/help/71-Prepopulating-fields-to-your-JotForm-via-URL-parameters

With this you can populate the field with the URL of your choice, and then hide the field using a conditional field:

http://www.jotform.com/help/196-How-to-Hide-or-Show-Multiple-Fields-at-Once-Based-On-a-Single-Conditional-Entry

Please let us know if there is anything else we can assist you with and we will be happy to help.

Thank you for using Jotform!