how to add scrollbar in dropdown menu

I want to add a scrollbar in dropdown menu here's the html

<ul class="dropdown-menu dropdown-scrollbar">

                            <!-- <li><a href="#"></a></li> -->
                               {% for table_name in obj %}
                                <option>{{ table_name.table_name }}</option>
                              {% endfor %}                          
                            <!-- <li><a href="#">Dropdown link</a></li> -->
</ul>     

i try this css but it not works *i know it is a duplicate question , but i try the css from the SQ answer and still not work , maybe there is something i missing?

.dropdown-scrollbar
{
    height: 50px;
    overflow-x: hidden;
    overflow-y: scroll;

}

thanks a lot