You would first need to modify the search server behavior on the results page. You will need to add a new search criteria to filter on the subCategory URL parameter that you are passing.
To do this you would need to open the search page and edit the current DataAssist Search server behavior that is on the page. In here you will need to create a new criteria that will look at the subcategory column and compare it to your new URL parameter.
If you set it up like this you should be able to get the sub category specified in the link by having the recordset filtered for that particular sub category.