Set the placeholder attribute of the input element to "Username".

<html>
<head><style>body {min-height: 350px;} input { font-size: 16px; } textarea { font-size: 16px; } </style></head>
<body>
  <img src="https://mimo.app/r/snapcat.svg">
  <h3>Create an Account</h3>
  <input type="text" placeholder="Username">
  <br>
  <input type="text">
  <br>
</body>
</html>]]>

Make sure to assemble placeholder="Username".