Popup opened by event. Android

I just want to made a registration form with username and password TextFields, and when the registration is successful i would bring up a popup window titled "Registration successful", and with a button to change scene (going a login scene). This is my code:

     var result:boolean; //this variable attests that registration has been successful
     var username = "";
     var password = "";
     
     function OnGUI ()
     {        
         
         username = GUI.TextField (new Rect (x, y, width, height), username);
         
         password = GUI.PasswordField (new Rect (x, y, width, height), password);
         
         if (GUI.Button (new Rect (x, y, width, height), "Sign In")) {
             
             signIn ();
             
             if (result) {
                 
                 /* I just want my PopupWindow here */
                 
             }
         }
     }
 
     var URL0 = "http://example.php";
  
     function signIn() {
     
         var form = new WWWForm();
         form.AddField("username", username);
         form.AddField("password", password);
         var w = WWW(URL0, form);
         yield w;
         
         if (w.error) {
             print(w.error);
         } else {
             result = w.text == "true" ? true : false;
             w.Dispose();
         }
         
     }

I tried to put a PopupWindow in there, but it does not appear!