XC5 should be using HTML5 out of the box and this is easily done with simple HTML5 rules. No js required.
Unfortunately I don't think XC5 is coded to take advantage of this so you will have to do this yourself via a custom module.
Basically it comes down to adding new <option> tag as first one in the <select> and setting its properties as below
Code:
<select>
<option disabled value="" selected>Please Select</option>
<option>1</option>
<option>2</option>
........
</select>
Here is live example -
https://www.cflsystems.com/site-structured-data-for-x-cart-5.html
It is on XC4 but the idea is the same