how to make a dropdownlist visible once clicking a button

Question

I am a beginner, Hope one can help me with my problem, I want to make a drop down list visible once I click the button, It completely like the thing that has been done here: https://shop.adidas.co.in/#category/Pag-60/No-0/0/All/ facet .DIVISION_CATEGORY_ss:%28%22FOOTWEAR%22%29%20AND%20allBrands_ss:%28%22ORIGINALS%22%29

when the customer clicks on Buy button then a drop-down list appear above of the buy button,I think this example in the mentioned website is clear enough,

EDITION

as the code shows, when I click on BUY button it opens up a new link, I'm not wanna to do this, at first when someone click the button it has to not work and just show the drop-down list, secondly, when the user chooses a size from the drop-down list and then click on the button it has to work and open the link,

it is exactly what happen in the above link that I have introduced. my stuff:

<button name="buySize" id="buySize" type="button" class="btn btn-success btn-sm" onclick="location.href='@Url.Action("AddToCart", "ShoppingCart", new { Barcode = @item.Barcode },"")'">
    خرید &raquo;
</button>
<div class="pull-left">
    <label class="control-label" id="drpSize">سایز:</label>
    <div class="product-quantity" id="drpSize" >
        @Html.DropDownListFor(model => model.CartItems[0].Size, new SelectList(Model.Sizes))
    </div>
</div>

this was my drop-down list and button and also javascript :`

$('#buySize').click(function () {
    if (this.click())
        $('#drpSize').show("fast");
    else
        $('#drpSize').hide("fast");
});

every help is appreciating :),

Answer 1

Try this :

1. Your dropdown is initially hide by the css ie. display:none

Jquery code to show it when user click on buy button:

$('#buySize').click(function () {
   $('#dropdown_id').show();
});

Answer 2

You can put drop down list hidden by default using style attribute in drop down div as

style="display:none"

on click of button you can show the drop down list. For this code will be as follow:

CSHTML

EDIT: You achieve required functionality First remove onclick event from button control.

 <button name="buySize" id="buySize" type="button" class="btn btn-success btn-sm">
  خرید &raquo;</button>
   <div class="pull-left">
     <label class="control-label" id="drpSize">سایز:</label>
      <div class="product-quantity" id="drpSize" style="display:none" >
       @Html.DropDownListFor(model => model.CartItems[0].Size, new SelectList(Model.Sizes))                                                              </div>
    </div>

in js file

 $('#buySize').click(function () {
           $('#drpSize').css('display','block'); 
        });

EDIT: alternately if want to show drop down with animated effect and want to open BUY link after size selection use this script.

 $('#buySize').click(function () {
   $('#drpSize').show('fast');
var val= $('#dropDownId :selected').text();
if(val!='Select'){
location.href= //which ever link you want to open put here
}
 });