JSFiddle - React, Tailwind, and code Playground

HTML

<select name="selection" class="box1" multiple>
    <option>0</option>
    <option selected>1</option>
    <option>2</option>
</select>
<br/>
<input type="text" name="hid" value="10" class="box2" />

JavaScript

$(".box1").change(function () {
    var selected = $(this).find("option:selected").text(++);
    $(".box2").val(selected);
});