How to make a scrollbar in bootstrap dropdown?

How to create a scrollbar in bootstrap 4 dropdown selection. In my case if the files is too much then the dropdown will not fully loaded. So I plan to fixed only 5 files to be displayed when the dropdown is clicked and have a scrollbar.

HTML

<div class="pd-ltr-20 customscroll customscroll-10-p height-auto xs-pd-20-10">
    <div class="min-height-200px">
        <div class="mb-30">
            <div class="row">
                <div class="col-sm-12">
                    <div class="form-group">
                        <small class="form-text text-muted">Select ES Value</small>
                        <div class="custom-file">
                            <select class="font-14 selectpicker form-control" data-width="100%">
                                <option value="">- ES VALUE -</option>
                            </select>
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>