Edit in JSFiddle

$.ajaxSetup({xhrFields: {withCredentials: true}});	// For cookies with SeqId

// Sending message
$('#sendBtn').click(function() {
  $.post(mcastUrl, $('#msgInput').val());
  $('#msgInput').val(null);
})

// Receiver A
var receiveA = function() {	
  $.get(mcastUrl)
    .done(function(data) {
      $('#receiverA').append(data + '<br>');
      receiveA();
    })
}

// Receiver B
var receiveB = function() {	
  $.get(mcastUrl).done(function(data) {
    $('#receiverB').append(data + '<br>');
    receiveB();
  })
}

var mcastUrl;
$('#channelInput').on('change keyup', function() {
  mcastUrl = 'https://httprelay.io/mcast/' + $('#channelInput').val();
  $('.mcastContainer').html(mcastUrl)
})

// Generating random channel id
$('#channelInput').val(Math.random().toString(36).substr(4)).trigger('change')

receiveA();
receiveB();
<fieldset>
  <legend>Communication channel ID (must be unique and secret)</legend>
  <label target="channelInput">http://httprelay.io/mcast/<label/><input id="channelInput"> 
</fieldset><br>

<fieldset>
<legend>Send information to POST <span class="mcastContainer"></span></legend>
  <input id="msgInput" placeholder="Message">
  <button id="sendBtn">Send</button>
</fieldset><br>

<fieldset id="receiverA">
  <legend>A receiver from GET <span class="mcastContainer"></span></legend>
</fieldset><br>

<fieldset id="receiverB">
  <legend>B receiver from GET <span class="mcastContainer"></span></legend>
</fieldset>
* {
    font-family: "Trebuchet MS", Helvetica, sans-serif;
    color: #555;
}