I believe you mean a throbber, as all content is preloaded before playback begins (Animate's misused terminology notwithstanding).
If there is no such publish option for WebGL, you'll have to manually edit the published output to include your animated element behind the stage.
It's even possible to create a proper progress indicator by sticking a load progress listener in with the other load listeners on the page, like this:
loader.addEventListener("progress", myLoadProgressHandler);
Where your load progress handler would be something like:
function myLoadProgressHandler(evt) {
var percentLoaded = evt.progress;
// display a progress bar or something
}