Edit: The solution below is best especially if you do not need any customization

I think you would need to add a custom element to the screen. something like this may work for you

call this when you make the ajax call

var iframeDiv = document.createElement('div');


   iframeDiv.id = "popupFrameDiv";


   document.body.appendChild(iframeDiv);


   document.getElementById('popupFrameDiv').innerHTML = '<div id="popupFrameDivBG" style="background-color: rgba(0, 0, 0, 0.8);position: fixed;width: 100%;height: 100%;top: 0;text-align:center;padding-top:20px;"><div style="border-radius:10px;padding:10px;width:500px;background:#fff;height:300px;margin: 0 auto;font-size:20px">Please wait while the data is loaded.</div></div>';

and this after you have gotten the data

document.getElementById('popupFrameDiv').parentNode.removeChild(elem)