JSFiddle - React, Tailwind, and code Playground

HTML

<form>
<textarea>initial content</textarea>
<input id=setval value="else">
<input type=reset>
</form>

<textarea id=output></textarea>
<button>get</button>
<button id=set>set</button>

CSS

textarea { width:300px;}

JavaScript

document.querySelector("button").addEventListener('click', (e) => {
	let o = document.querySelector('#output');
  let i = document.querySelector('textarea');
  o.value = "Value: " + i.value + "\nDef.Value: " + i.defaultValue;
});

document.querySelector("#set").addEventListener('click', (e) => {
  let i = document.querySelector('textarea');
  i.defaultValue = document.querySelector('#setval').value;
});