R – In AIR how to set focus on textinput on application load

You need to wait for the flex container to be registered with the display list so you access the stage.

Put a call to init from you creationComplete handler:

<mx:Script>
    <![CDATA[
        import flash.events.Event;

        private function init():void 
        {
            addEventListener(Event.ADDED_TO_STAGE, initScreen, false);

        }

        private function initScreen(e:Event):void 
        {
            removeEventListener(Event.ADDED_TO_STAGE, initScreen);
            stage.focus = userName;
        }

    ]]>
</mx:Script>

<mx:Form defaultButton="{enterBtn}">

    <mx:FormHeading label="Form" />
    <mx:FormItem label="Username" tabIndex="1">
        <mx:TextInput id="userName" text="" selectionBeginIndex="0" />
    </mx:FormItem>
    <mx:FormItem label="Password" tabIndex="2">
        <mx:TextInput displayAsPassword="true" id="password"/>
    </mx:FormItem>

</mx:Form>

I try to avoid using the AddChild state tag. It's usually better to put all that in a component, and use SetProperty to set visible and includeInLayout when you want it to display.

You can always override visible in your custom component to set the focus to the field. Or create a custom setter than does the same thing

public function set show(value:Boolean):void
{
  visible = true;
  includeInLayout = true;
  if (value)
  myFunctionThatSetsTheFocus();
}