<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<script>
$(document).ready(function(){
  $("#sel1").change(function(){
    $("#target").val($(this).val())
  });
});
</script>
<div class="form-group">
  <label for="sel1">Select list:</label>
  <select class="form-control" id="sel1">
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
  </select>
</div>
<input type="text" id="target">