<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/dist/css/bootstrap.min.css" rel="stylesheet">

<form action="/wp-content/custom-forms/register.php" method="POST" class="container mt-4" style="max-width: 400px;">
  <input class="form-control mb-2" type="text" name="username" placeholder="Username" required>
  <input class="form-control mb-2" type="email" name="email" placeholder="Email" required>
  <input class="form-control mb-2" type="password" name="password" placeholder="Password" required>
  <button class="btn btn-primary w-100" type="submit">Register</button>
</form>