http://www.9rays.net/asp.net_2/treeview/Doc_DragDrop.aspx
See flytreeview_dragObject.
So the code should look like the following:
if (flytreeview_dragObject) {
flytreeview_dragObject.defaultMode = 1;
// 1 - move,
// 2 - copy,
// 3 - defined by Ctrl (move default),
// 4 - defined by Ctrl (copy default).
flytreeview_dragObject.update();
}