Add this code on a HTML widget to activate preloader. You have to insert your own icon/image URL. See example below.
The HTML widget has to be the first one to load. Try to put it on the top of the page or header and see how it goes. And remember to change the background color to match your site’s.
Here’s a GIF icon for you. Feel free to download and use it for your sites. If you want to make your own icons go to this site.