Define Value from Select Option using Ajax

<script>
    $(document).ready(function(){
        $("#your_select_id").change(function(){ //select box change function event
            var new_variable = $("#your_select_id").val(); // create new variable and get the value from your select box
            $.ajax({ //ajax
                url: "your_process_page.php", //link to your process page
                data: "new_variable="+new_variable, //which data will transfer to process page, dont forget to add $_GET['new_variable']; in your process page
                cache: false,
            })
            .done(function(ajaxresults){
                $("#your_defined_input").val(ajaxresults); //make your defined input to set its value depend to result of the process before
            });
        });
    });
</script>