Hi Jon!
You can use our API to register the javascript module 'Loader' on your page. For that you'd use
ScriptHelper.RegisterLoader(Page);
You'll then need to make the loader appear. If properly registered the loader should be available in the Javascript variable
window.Loader
and supports calls like
window.Loader.show();
window.Loader.hide();
However I'd like to suggest using this C# method as well:
ScriptHelper.RegisterPageLoadedEvent(Page);
Internally this method registers a Javascript which is rendered at the end of the request. The Javascript code enables the 'pageLoaded' flag and fires PageLoaded Event on the EventHub. This hides the Loader.
Your approach depends on how you're handling all the other things your application does during the long running task.
One last note: I have to tell you, that JavaScript API is not what we consider our public API and it may change without notice. During upgrade or hotfix. However unlikely it is to change 'show' and 'hide' commands of a Loading indicator, I had to tell you.