public function init():void
{
    txtBox.addEventListener(KeyboardEvent.KEY_UP,pressEscape);
}

public function pressEscape(event:KeyboardEvent):void
{
    if (event.keyCode==13)
    {
        Alert.show("Enter Key has been pressed");
    }
}

Use this code to handle keypress/keyboard events

KeyPress/keyboard events handling in Flex AS3

actionscript, event, Flex

ActionScript 3