when select a option from dropdown open a divsion code example

Example 1: open select dropdown

$('#MyButton').on("click",function(){
    $("#selecttest").attr("size", 8);
    $("#selecttest").css("position","fixed");
    $('#MyButton').css("margin-left", $("#selecttest").width() + 5);});
$('#selecttest').on("change",function(){
    $("#selecttest").attr("size", 1);});

Example 2: open select dropdown

var myDropDown=$("#myDropDown");
var length = $('#myDropDown> option').length;
//open dropdown
myDropDown.attr('size',length);