jquery.dropdown.js required not working

im using the jquery.dropdown.js plugin. my problem is there is no notification appear when submitting the form, leaving the required select tag with no value.

html

<div class="dropdown-sin-1" style="margin-top: -15px;">
        <select name="" placeholder="" required>      
            <option value="" disabled selected></option>
            <option value="1">1</option>
            <option value="2">2</option>
            <option value="3">3</option>
            <option value="4">4</option>
            <option value="5">5</option>
            <option value="6">6</option>
            <option value="7">7</option>
        </select>
        </div>

js

$('.dropdown-sin-1').dropdown({
  readOnly: false,
  limitCount: Infinity,
  input: '<input type="text" maxLength="20" placeholder="Search">',
  data: [],
  searchable: true,
  searchNoData: '<li style="color:#ddd">No Results</li>',
  choice: function () {}
});