Assign onChange Event for Ajax 4.0 ComboBox

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ASP.NET

Dear All,
I am using below code to create Ajax ComboBox dynamically and I want to assign onchange event using javascript for comboBox. Even I am assigning onchange event but nothing happens.

AjaxControlToolkit.ComboBox cmb = new AjaxControlToolkit.ComboBox();
cmb.RenderMode = AjaxControlToolkit.ComboBoxRenderMode.Block;
cmb.Attributes.Add("onchange", "javascript:alert(this);");
....

I would appreciate to know how to assign onchange event "SelectedIndex Change Event" using javascript?

Thanks.