Step 1
Below is some custom CSS that you need to add to the page <header> code. You can access the code via the Designer.
CUSTOM CSS
Step 2
Add the following code to your page before the closing <body> tag.
I recommend copying the code on the published site to preserve line breaks.
<!--🟢 DATE PICKER CODE 🟢-->
<script src="https://fengyuanchen.github.io/datepicker/js/datepicker.js"></script>
<script>
  $(document).ready(function () {
    $('[data-input="datepicker"]').datepicker({
      format: 'mm-dd-yyyy'
    });
    // Available date placeholders:
    // Year: yyyy
    // Month: mm
    // Day: dd
  });
</script>
Credits
I discovered this code on the Flowbase blog.