JSFiddle - React, Tailwind, and code Playground

HTML

<form>
 <input name="val3" id="val2" />
    <select name="val1" id="val1">
        <option value="">Select a value</option>
        <option value="foo">foo</option>
        <option value="bar">bar</option>
    </select>
  
   
</form>

JavaScript

$('#val1').change(function () {
    changeThird();
});



function changeThird() {
    $('#val2').val($('#val2').val() + ' ' + $('#val1').val());
}