<Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="3*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <TextBlock Grid.Row="0"
                   Grid.Column="0"
                   Margin="5,5,5,0"
                   FontSize="14"
                   HorizontalAlignment="Right"
                   Text="User:">
        </TextBlock>
        <TextBox  Grid.Row="0"
                  Grid.Column="1"
                  Margin="5,5,5,0"
                  Name="_txtLogin" 
                  Text="{Binding Login}"/>
        <Button Grid.Row="1"
                Grid.Column="1" 
                Content="Ok"
                Width="100" 
                VerticalAlignment="Center" 
                HorizontalAlignment="Center" 
                Click="AutentificateClick" 
                Padding="0,2,0,2" 
                IsDefault="True" 
                Name="_btnOk" 
                />
    </Grid>

private Boolean _isFirstTime = true;

        public Window2()
        {
            InitializeComponent();
            DataContext = this;
        }

        private void AutentificateClick(Object sender, RoutedEventArgs e)
        {
            _btnOk.IsEnabled = false;
            Cursor = Cursors.Wait;

            Task<Boolean>.Factory.StartNew(InitConnection).ContinueWith(t =>
            {
                if (!t.Result)
                    return;

                // Emulate some work after connection's been established
                Thread.SpinWait(1000000000);
                
                _btnOk.IsEnabled = true;
                Cursor = Cursors.Arrow;

            }, TaskScheduler.FromCurrentSynchronizationContext());
        }

        private Boolean InitConnection()
        {
            Thread.SpinWait(10000000);
            return true;

            if (_isFirstTime)
            {
                // Emulate some work to establish connection
                Thread.SpinWait(10000000);
                _isFirstTime = false;
            }
            
            return true;
        }

        Thread.SpinWait(10000000);
        return true;