How to run client script of dropdownlist

How to run client script of dropdownlist

The client-side event name is "onchange", e.g.
ddl_specs.Attributes.Add ("onChange", "OnChangeSpec();");

SelectedIndexChanged is the server-side event which can be used as this:
ddl_specs.SelectedIndexChanged +=new
EventHandler(ddl_specs_SelectedIndexChanged);