I could not find in the documentation an explanation for the callbackFnName parameter of RadWindowManager.RadAlert.
It works for me when I have null there, but I am just guessing.
you need to get a reference to the radalert object and then use the
ASP.NET AJAX add_close() function to hook to its close event. Here is a sample code.
We recently introduced callback function for radalert and it works just as the ones of radconfirm and radprompt. The parameter has the name of a client - side function which will get executed just after the alert is closed. In addition, by checking the argument there you can determine whether the alert was closed through the OK button or the [X] button.
More detailed explanation is available in the Description tab of the demo below: