Passing Parameters from one view to another using link_to

I am currently trying to populate hidden fields on a form based upon what values are present on the current form I am working on.

For example, when I am viewing info for category with an id of 3 I have access to the parent_id of 2. If I click the new link, I want to populate the parent_id of 2 to a hidden field parent_id on the view. I can see how to add the query string no problem, how do I populate the hidden field. I have just done 2-3 hours of searching and can find nothing useful. I would think this would be a common issue.