Attaches a handler to an event in an EventHandlerList instance and adds the event to the list if it is not already present.
Use the addHandler method to programmatically attach handlers to events in Microsoft ASP.NET AJAX script components. In the ASP.NET AJAX component event model, client events reside in an EventHandlerList object. Handlers are created and assigned to the event by using the addHandler method. You then raise the events by calling the method that is returned by the getHandler method.
If the id value that you pass to addHandler is already in the EventHandlerList instance, the method specified by handler is added to any handlers already associated with the event. Otherwise, a new event that has the specified ID and handler is added to the EventHandlerList object.