Add a WordPress contact form without a plugin or SMTP setup

<form action="https://formthunder.com/submit" method="POST">
  <input type="hidden" name="access_key" value="YOUR_ACCESS_KEY">
  <input type="hidden" name="subject" value="New message from the website">
  <input type="hidden" name="redirect" value="https://yoursite.com/thank-you/">
  <input type="checkbox" name="botcheck" style="display:none" tabindex="-1" autocomplete="off">

  <label for="name">Name</label>
  <input id="name" name="name" required>

  <label for="email">Email</label>
  <input id="email" name="email" type="email" required>

  <label for="message">Message</label>
  <textarea id="message" name="message" rows="5" required></textarea>

  <button type="submit">Send</button>
</form>

add_shortcode('formthunder_contact', function () {
    ob_start(); ?>
    <form action="https://formthunder.com/submit" method="POST">
      <input type="hidden" name="access_key" value="YOUR_ACCESS_KEY">
      <input type="hidden" name="redirect" value="<?php echo esc_url(home_url('/thank-you/')); ?>">
      <input type="checkbox" name="botcheck" style="display:none" tabindex="-1">
      <p><label>Name <input name="name" required></label></p>
      <p><label>Email <input name="email" type="email" required></label></p>
      <p><label>Message <textarea name="message" required></textarea></label></p>
      <p><button type="submit">Send</button></p>
    </form>
    <?php return ob_get_clean();
});