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>