The buttons use the 'onclick' event to trigger their action. The dropdown list has an 'onchange' DOM event which you can use. If you are using an ASP.NET dropdown then remember to set AutoPostback to false.
An idea here would be to use a script to generate the sent parameter, fx. extract the value of the currently selected item. You can tell the control that the passed parameter is a script by setting the isArgumentScript parameter to true. This means that the argument is rendered outside quotes and gets run every time the event is triggered.