How to add a loading page icon on this theme

Hello,
I’ve created a page with many Facebook pages events listed. I’ve created it with Custom Facebook feed plugin.
Since the page take sometime to load, I would like to know how I can add a loading gif icon in your theme before the page completely load.

For example, I got this code (https://smallenvelop.com/display-loading-icon-page-loads-completely/) on internet that explain how to add the loading icon, but I don’t know on which pages of the theme to add them.

Please, can you help me with this?

I also know that wordpress have its Native WordPress Loading GIFs (https://austin.passy.co/2014/native-wordpress-loading-gifs/), but how can I use them in your Quest theme?

Thank you