I tried to get the ID on the option selected when the modal was in show state.
$("#modal-form").on('shown.bs.modal', function(e) {
  console.log("CHANGE");
  var id = $("#customer option:selected").val();
  console.log(id);
});<div class="form-group">
  <label for="customer" class="col-md-2 control-label">Customer</label>
  <div class="col-md-9">
    <select class="form-control" id="customer" name="customer" class="form-control">
      <option value="John">John</option>
      <option value="Alex">Alex</option>
    </select>
    <span class="help-block with-errors"> </span>
  </div>
</div> 
     
     
     
    