JSFiddle - React, Tailwind, and code Playground

HTML

<div class="button transparent_2">
    <a class="extra-color-3" href="#">Get a quote</a>
</div>

<p id="result"></p>

JavaScript

var $a = document.getElementsByClassName("transparent_2");
    $a[0].className += " fancy-btn";

document.getElementById("result").innerHTML = $a[0].className;