The required flag is not on the <mx:Form> itself, but actually on the <mx:FormItem>
<mx:FormItem> is a type of container so I'm pretty sure you can try something like this:
<mx:FormItem label="Test" required="true">
I should have mentioned <mx:FormItem> instead of <mx:Form>.
So question is: Is it possible to have an asterisk around a label of a label-textinput pair which are not in a form/formitem?
Yes, you can put it there yourself. I use something like this.
<s:Group id="indicator" top="0" bottom="0">
<s:BitmapImage y="5" fillMode="clip" x="0"