Calling AppleScript from Swift App, passing a parameter. Basic

gistfile1.txt

import Carbon

//guard let url = Bundle.main.url(forResource: "SendFinderMessage", withExtension: "scpt") else {
//    exit(0)
//}
//guard let script = NSAppleScript(contentsOf: url, error: &errors) else {
//    exit(0)
//}


var errors: NSDictionary?

let script = NSAppleScript(source: "on show_message(message)\n tell app \"System Events\" to display dialog message\n return \"Cool\"\n end")

let handler = NSAppleEventDescriptor(string: "show_message")

let message = NSAppleEventDescriptor(string: "Message from App")

let parameters = NSAppleEventDescriptor.init(listDescriptor: ())
parameters.insert(message, at: 1)


let event = NSAppleEventDescriptor.appleEvent(withEventClass: AEEventClass(kASAppleScriptSuite), eventID: AEEventID(kASSubroutineEvent), targetDescriptor: nil, returnID: AEReturnID(kAutoGenerateReturnID), transactionID: AETransactionID(kAnyTransactionID))

event.setParam(handler, forKeyword: AEKeyword(keyASSubroutineName))
event.setParam(parameters, forKeyword: AEKeyword(keyDirectObject))


let returnedDescriptor = script?.executeAppleEvent(event, error: &errors)

returnedDescriptor?.stringValue
errors

where does keyASSubroutineName come from? I'm getting an error about it being undefined in the scope