JSFiddle - React, Tailwind, and code Playground

HTML

<select class="myDropdown">
 <option value="volvo">Volvo</option>
 <option value="saab">Saab</option>
 <option value="mercedes">Mercedes</option>
 <option value="audi">Audi</option>
 <option value="Regular">Regular</option>
</select>

<input class="quantity" type="text" />

JavaScript

$('.myDropdown').change(function () {
    var currentDropdownValue = $(this).val(); 
    if (currentDropdownValue == "Regular") {
         $(this).next('.quantity').val('10');
    }
});