Often you need to access the ScriptManager on a Content Page to e.g. set Focus to a Button. The easiest way to do this is using the static GetCurrent method of the ScriptManager Class. Like this:
ScriptManager.GetCurrent(Page).SetFocus(btnSubmitRequest);
Unless you're really sure a ScriptManager exists, you have to check if it exists like this:
// Does a ScriptManager exists for the page?
ScriptManager scMan = ScriptManager.GetCurrent(Page);
if (scMan == null) throw new Exception("No ScriptManager found!");