Your HTML
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<form action="https://api.pathline.io/submit"
      method="POST">
  <input type="hidden" name="access_key"
         value="YOUR-KEY">
  <input type="text" name="name" required>
  <input type="email" name="email" required>
  <input type="tel" name="phone">
  <textarea name="message"></textarea>
  <button type="submit">Send</button>
</form>

form.addEventListener('submit', async (e) => {
  e.preventDefault();
  const res = await fetch('/submit', {
    method: 'POST',
    body: new FormData(form),
  });
});

<form onSubmit={handleSubmit}>
  <input name="access_key"
         value="YOUR-KEY" />
  <input name="name" required />
  <input name="email" type="email" required />
  <input name="phone" type="tel" />
  <textarea name="message" />
  <button type="submit">Send</button>
</form>