select2-basique.html

<select name="musique" class="select2">
  <option value="1">Guitare</option>
  <option value="2">Harmonica</option>
  <option value="3">Piano</option>
  <option value="4">Flute</option>
  <option value="5">Violon</option>
  <option value="6">Trompette</option>
  <option value="7">Saxophone</option>
  <option value="8">Batterie</option>
  <option value="9">Accordéon</option>
</select>

<script
  src="https://code.jquery.com/jquery-3.4.1.min.js"
  integrity="sha256-CSXorXvZcTkaix6Yvo6HppcZGetbYMGWSFlBw8HfCJo="
  crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.9/js/select2.min.js"></script>
<script>
// In your Javascript (external .js resource or <script> tag)
$(document).ready(function() {
    $('.select2').select2();
});
</script>