JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <p><select id="select">
      <option>Пункт 1</option>
      <option>Пункт 2</option>
      <option>Целевой пункт 3</option>
    </select></p>
    <p><textarea id="textarea" rows="10" cols="50"></textarea></p>
  </form>

JavaScript

function onchange1(e) {
  
        document.getElementById("textarea").value = this.selectedIndex == 2 ? "Нужное значение" : "";  
    
}

document.getElementById('select').onchange = onchange1;