How to make the Category sticky in the directory?

We have navigation from other places on our site into the business directory that require a category filter. For instance, if you click on "Find and Attorney", it goes to the directory and pulls up the attorneys (Category filter = Attorneys). Now if I type something in the search, it finds all my listings with that word... Attorneys, Florists, CPA's.. all the listings regardless of category. I want to make the category "sticky" so if I got to the directory using the "Find and Attorney" button, it will apply this category filter for "Attorneys" in addition to my search terms. How can I do that?