Accessing a class private variable in the main timeline

Greetings,
noob question.

Here’s my main class:

package 
{
    import flash.external.ExternalInterface;
    
    public class main
    {

        public function main()
        {
        }

        public function check_DVBViewer()
        {

            ExternalInterface.call("check_DVBViewer","test");
            ExternalInterface.addCallback("AutoIT",AutoIT);

        }

        private function AutoIT(my_msg:String)
        {
                // here we get the callback string
        }

    }

}

in the timeline I have a dynamic text box named my_textbox:

var my_test:main = new main();
my_test.check_DVBViewer();
my_textbox.text = ???????

Basically the class waits for the callback variable (my_msg).
The problem is: I need to set the dynamic textbox text to my_msg variable.

How can I access it?
I know I can access a public var in the timeline with functioname.variable but I can’t define a public var inside a function, right?

Thank you very much for your time.