If you want to add an extra search field, then you have to add it to the template first. You can use this filter to load a custom template ( you can use the /frontend/templates/default.php as a starting point ).
Then you have to include the data for the extra field in the function makeAjaxRequest in the /js/wpsl-gmap.js ( this will be easier in the future. I’m planning to make the js script automatically grab the value from all form fields that are present and include it in the ajax params. ).
The /js/wpsl-gmap.min.js is the one used on the front-end, so you do have to minify the content of /js/wpsl-gmap.js on a site like this, or enable https://codex.www.ads-software.com/Debugging_in_WordPress#SCRIPT_DEBUG so it loads the /js/wpsl-gmap.js instead of the /js/wpsl-gmap.min.js.