JSFiddle - React, Tailwind, and code Playground

HTML

<!--フォームを表示するボタン-->
<button onClick="form_open();" type="button" class="btn">追加</button>

<!--フォーム-->
<div id="form" style="display: none;">
 <form method="post" action="">
  <h2>情報追加</h2>
   <textarea name="data"></textarea>
   <input type="submit" class="btn">
   <button type="button" onClick="form_close();" class="btn">閉じる</button>
 </form>
</div>

JavaScript

function form_open(){
 document.getElementById("form").style.display = "block";
}

function form_close(){
 document.getElementById("form").style.display = "none";
}