Selecting Multiple in Dropdown

So I'm making a Drop Down Multiple Select but I don't know how to properly change the scripts or the function. Here the multiple select here is the code for the script or function:

       function show_subj(){
        var studID = $('#ddl_stud').val();
        console.log(studID);
        if(studID){
            $.ajax({
                type:'POST',
                url:'grade_dropdown.php',
                data: 'stud_id='+studID,
                success:function(html){
                    $('#ddl_subj').html(html);
        console.log($('#ddl_subj').html(html));
                }
            }); 
        }
    }