Edit in JSFiddle

var token = document.getElementById('token');
var message = document.getElementById('message');
var button = document.getElementById('notify');

function sendMessage() {
  fetch('https://mozcurius.herokuapp.com/notify', {
    method: 'POST',
    mode: 'cors',
    headers: {
      'Content-type': 'application/json'
    },
    body: JSON.stringify({
      token: token.value,
      client: 'JSFiddle',
      payload: {
        title: 'Message from the bottle',
        body: message.value
      }
    })
  });
}

button.onclick = sendMessage;

<p>
  token:
  <input type="text" id="token">
</p>
<p>
  message:
  <input type="text" id="message">
  <button id="notify">
    Notify Me!
  </button>
</p>
<p>
  Get your token at <a href="http://mozcurius.herokuapp.com/">Mercurius site</a>.
</p>