form.Focus();

<StackPanel Name="uiStack" KeyDown="uiStack_KeyDown">
        <Button Name="uiButton" Click="uiButton_Click"/>
    </StackPanel>

private void uiStack_KeyDown(object sender, System.Windows.Input.KeyEventArgs e)
        {
            // Do some thinks here
        }

        private void uiButton_Click(object sender, RoutedEventArgs e)
        {
            uiStack.Focusable = true;
            uiStack.Focus();
        }