To add a pre-loader SVG until the flutter web app is loaded you can include this JS in Custom Headers under Web Deployment .
<script type="application/javascript" src="https://cdn.jsdelivr.net/gh/jasminder/flutter_preloader/preloader.js" defer></script>
Moreover, you can clone the repo and change SVG/image as per your requirement and use jsdelivr as cdn for your js.
Please post in comments if you have any queries