ESP3D-WEBUI/extensions_samples/dispatch.html

<script type="text/javascript">
  function sendMessage(msg){ 
      window.parent.postMessage(msg, '*');
  }

  function processMessage(eventMsg){
        if (eventMsg.data.type  && (!eventMsg.data.id||eventMsg.data.id=="panel")){
            if (eventMsg.data.type=="dispatch"){
                const line =  eventMsg.data.content
                const resultPanel = document.getElementById("output");
                resultPanel.innerHTML = resultPanel.innerHTML + "<br> this messsage is from "+ line.initiator.id+ " <span class='text-primary'>"+ line.response+"</span><br><hr />";
                           
            }
        }
    }

  window.onload = (event) => {
    window.addEventListener("message", processMessage, false);
  }
    
</script>

<div class="container">
    <button class="btn m-1" onclick="sendMessage({type:'dispatch', target:'webui', id:'panel', targetid:'panel',content:'this is a broadcasted message'});">Dispatch</button>
    <div class="container m-2" id="output">
    </div>
</div>