Pass data from class to main thread?

Question

WPF

string save_proxy = textBox.Text;
            Main save = new Main();
            save.save(save_proxy);

This is how data is passed to the class, but how back?
You can create a global variable in the class and then refer to it and take the data, but this is not correct as far as I know.
It is necessary to transfer data from the class to the main thread.