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<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script defer="defer" src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.10/js/select2.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.10/css/select2.min.css" rel="stylesheet">

<select id="mySelect2">
  <option value="apple">apple</option>
  <option value="orange">orange</option>
  <option value="banana">banana</option>
</select>

$(document).ready(function() {
  $("#mySelect2").select2({width: "100%"});
  // valの引数で指定されたものが選択された状態で表示される
  $("#mySelect2").val("orange").trigger("change");
});

参照URL:
https://r17n.page/2019/09/01/jquery-select2-initialize-and-clear/