To get a reference to the RadXmlHttpPanel control, use the ASP.NET AJAX's native $find() method:
var panel = $find("<%=RadXmlHttpPanel.ClientID %>");
Note, that this object is available in the Sys.Application.Load event at the earliest.
Once you have the client-side object of the control, you can use its client-side API.
The control offers a number of client-side events to let the developer apply the logic needed by the current project.