JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="texto" id="texto"></textarea>
<button type="button">Assinar</button>

CSS

button {
    display: block;
}
textarea {
	height: 200px;
	width: 200px;
}

JavaScript

var textarea = document.querySelector('textarea');
var button = document.querySelector('button');
var assinatura = '\n\nCumprimentos,\nSuper heroi.'

button.addEventListener('click', function() {
    textarea.value += assinatura;
});