Sys.WebForms.InitializeRequestEventArgs postBackElement Property
Gets the postback element that initiated the asynchronous postback.
To get or set property values for client API properties, you must call property accessor methods that are named with the get_ and set_ prefixes. For example, to get or set a value for a property such as cancel, you call the get_cancel or set_cancel methods.
var elem = arg.get_postBackElement();
An HTML DOM element.