JSFiddle - React, Tailwind, and code Playground

HTML

<select id="Codes" name="CODEOPTION">
        <option value="Option A">Option A</option>
        <option value="B">Option B</option>
        <option value="C">Option C</option>
        <option value="D">Option D</option>
        </select>

<input type="text" name="CODES" value="">

JavaScript

$("#Codes").change(function(){
    $('input[name="CODES"]').val($(this).val()+$(this).val());

});