test bs

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.2.1/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.13.5/js/bootstrap-select.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.2.1/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.13.5/css/bootstrap-select.min.css">
 <select id="myselect" class="selectpicker" data-live-search="true" placeholder="please type" data-live-Search-Placeholder="search">
   <option value="111">AAAAAAAAAAA</option>
   <option value="222">bbbbbbbbbbb</option>
   <option value="333">cccccccccc</option>
</select>

JavaScript

$(function(){
   var select1 = $('#myselect').selectpicker();
	//var mysearchkeyword = $('#myselect').selectpicker('search.initiated').val();
	//var mysearchkeyword = $('#myselect').selectpicker('search.input').val();
	var mysearchkeyword = $('#myselect').selectpicker('.bs-searchbox').val();

   select1.on('changed.bs.select', function (e) {
    alert(mysearchkeyword);
    
   });
})