private void Button_Click_1(object sender, RoutedEventArgs e)
{
    textBox.Text = Random(str);
}

public string Random(StringBuilder str)
{
    str.Clear();
    for (int i = 0; i < 2; i++)
    {
        str.Append(((char)rnd.Next(48, 57)).ToString() +
                   (char)rnd.Next(65, 90) +
                   (char)rnd.Next(97, 122));
    }
    return Convert.ToString(str);
}