Updating an input filed from my class

1

I want to update a text input from my class the imput tex is called Name_txt. I get an undefined property error

package {
import flash.display.;
import flash.events.;
import fl.transitions.*;
import flash.display.MovieClip;

public class qwertyKeyboard extends MovieClip

{
    
    

// constructor
    
public function qwertyKeyboard()
    {
        
    this.addEventListener(MouseEvent.MOUSE_DOWN, addLetter);
    
        
    }
    
    
    public function addLetter(event:MouseEvent):void {
        
        
        
        trace(event.target.name.substr(0,1));
        Name_txt.text="hello";
        Name_txt.border =true;
        
       


        
 

    }
    
}

}