JSFiddle - React, Tailwind, and code Playground

HTML

<select name="myoption" id="myoption">
    <option value="1">A</option>
    <option value="2">B</option>
    <option value="3">C</option>
</select>

<input type="text" name="textbox" id="textbox" />

JavaScript

window.onload = function() {
    document.getElementById('myoption').onchange = function() {
        var text = this.options[this.selectedIndex].innerHTML;
        document.getElementById('textbox').value = text;
    };
};