How do I add a drop-down list to a dialog box?

I want to add a drop down list to this dialog box (script already available on this forum).

The drop down list content should contain say:

A

B

C

If user selects A, clicking on the Execute button displays message corresponding to A and same for others.

LaunchPalette();

function LaunchPalette()
{
	var resource =
		"""palette {
			
              properties:{ closeButton:true, maximizeButton:false,
				minimizeButton:false, resizeable:true },
			
              orientation:'row', spacing:15, margins:10,
              
              helloButton: Button { 
                  text:'Say Hello', alignment:['fill','fill'] },
             
              closeButton: Button { 
                  text:'Close', alignment:['fill','fill'] },
        }""";   
    
    var palette = new Window (resource, "My Palette");
    
    palette.layout.layout(true);
    palette.show();
    palette.bounds = {x:100, y:100, width:158, height:38};
    palette.layout.resize();

    palette.helloButton.onClick = function() 
    {
        alert("Hi there! You are using FrameMaker version "
            + app.VersionMajor + "." + app.VersionMinor + "." + app.VersionRevision
            + ".\n\nYour username/login is \"" + app.UserLogin + "\"."
            + "\n\nThe product is installed at the following location:\n\n  "
            + app.HomeDir
            + "\n\nHave a nice day!");
	};

    palette.closeButton.onClick = function() 
    {
        palette.close();
        palette = null;
	};

    palette.onClose = function() 
    {
        palette = null;
	};


    palette.onResize = function() 
    {
        palette.layout.resize();
	}
 }