Define a placeholder for the text area by coding placeholder.

<html>
<head><style>body {min-height: 300px;} input { font-size: 16px; } textarea { font-size: 16px; } </style></head>
<body>
  <img  src="https://mimo.app/r/profile.png" height="45">
  <textarea rows="3" cols="18" placeholder="What's on your mind?"></textarea>
</body>
</html>]]>

Make sure to code placeholder.