Select2 demo

HTML

<script src="https://rawgit.com/select2/select2/master/dist/js/select2.js"></script>
<link rel="stylesheet" href="https://rawgit.com/select2/select2/master/dist/css/select2.min.css">
<!DOCTYPE html>
<html lang="en">
<head>
  <title>Bootstrap Example</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/css/bootstrap.min.css">
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
  <link href="https://rawgit.com/select2/select2/master/dist/css/select2.min.css"/>
  <script src="https://rawgit.com/select2/select2/master/dist/js/select2.js"> </script>
  <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/js/bootstrap.min.js"></script>

</head>
<body>

<div class="container">
  <h2>Modal Example</h2>
  <!-- Trigger the modal with a button -->
  <button type="button" class="btn btn-info btn-lg" data-toggle="modal" data-target="#myModal">Open Modal</button>

  <!-- Modal -->
  <div class="modal fade" id="myModal" role="dialog">
    <div class="modal-dialog">
    
      <!-- Modal content-->
      <div class="modal-content">
        <div class="modal-header">
          <button type="button" class="close" data-dismiss="modal">&times;</button>
          <h4 class="modal-title">Modal Header</h4>
        </div>
        <div class="modal-body">
          <p>Some text in the modal.</p>
          <select class="jsSelect" name="states[]" multiple="multiple">
                                <option value="example1">example1</option>
                                <option value="example2">example2</option>
                                <option value="example3">example3</option>
                                <option value="example4">example4</option>
                                <option value="example5">example5</option>
                                </select>
        </div>
        <div...

JavaScript

$(document).ready(function() {
        
        $('#myModal').on('shown.bs.modal', function (e) {
  			$('.jsSelect').select2({
            placeholder: 'Select an option'
        });
			});
    });