Confirm box with event NodeDrop of RadTreeView

[Huy]

I have a page like this example : http://www.telerik.com/support/code-library/drag-and-drop-between-radgrid-and-radtreeview

I want to show a confirm box when user drop an item from radgrid or radtreeview to radtreeview from code behind with event NodeDrop of treeview.

So i do as this example for implement Radconfirm : http://www.telerik.com/support/code-library/radconfirm-from-code-behind

But it not working, radconfirm not show, i try to move code lines: 

string radalertscript = "<script language='javascript'>function f(){callConfirm(); Sys.Application.remove_load(f);}; Sys.Application.add_load(f);</script>";       
Page.ClientScript.RegisterStartupScript(this.GetType(), "radalert", radalertscript);

from event NodeDrop to Page_Load. Radconfirm show on pageload.

How to make it show in event NodeDrop of treeview