Hello JUAN CRESPO,
One option would be to do a callback from the OnClientDropDownClosing event handler by using the RadAjaxManger functionality (more information on the approach here
). This way you could avoid the actual postback.
On the other hand, if you wish to do a postback after the drop down has closed, you could use another approach: create an invisible asp Button which has its OnClick event handler on the server; hook to the OnClientDropDownClosed client event of the RadComboBox control and call the afore-mentioned button's click function, which will trigger the server-side event.
I hope this information is useful.
the Telerik team