If you have a Directories page that is set to display default results, you can create links to that Directories page which include variables that further refine the results. Any field included on the search form can be used whether it is hidden or not, provided it doesn't have a Default Value identified.
For the field you wish to filter on, use the Salesforce Name. This name can be viewed in the Directories page's Search tab in the Element column.
For an example of the formatting for the full url, review this link that filters our Portfolio page:
This link opens our Portfolio page and displays only the matching records where Project Type is "Non-Profit Soapbox". You may add additional values to the filter like this: