JSFiddle - React, Tailwind, and code Playground

HTML

<select id="select" name="options">
    <option>Choose Your Option</option>
    <option value="1"> 1</option>
    <option value="2"> 2</option>
    <option value="3"> 3</option>
    <option value="4"> 4</option>
</select>

<input id="btn" type="button" value="clickme">

JavaScript

var btn = document.getElementById('btn');
var select = document.getElementById('select');

btn.addEventListener('click', function(e) {
    btn.value = select.value;
});