JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="your-message" cols="40" rows="10" class="wpcf7-form-control wpcf7-textarea" id="ass" aria-invalid="false"></textarea><br>

Sposób HTML:
<button onclick="addText(this)">Button HTML 1</button>
<button onclick="addText(this)">Button HTML 2</button>
<button onclick="addText(this)">Button HTML 3</button>

<br/>

Sposób JS:
<button class="button-js">Button JS 1</button>
<button class="button-js">Button JS 2</button>
<button class="button-js">Button JS 3</button>

<br/>

<button onclick="clearText()">Clear</button>

JavaScript

const ass = document.getElementById('ass');

const addComma = () => ass.value ? ', ' : ''

// Sposób HTML
const addText = el => ass.value += addComma() + el.innerHTML;

// Sposób JS
const buttonsList = document.getElementsByClassName('button-js');
Object.values(buttonsList).forEach(el =>
	el.addEventListener('click', () => ass.value += addComma() + el.innerHTML)
);

const clearText = () => ass.value = ''