public partial class MainWindow
{
  public MainWindow()
  {
    InitializeComponent();
    DataContext = new Command();
  }
}
class Command : ICommand
{
  public void Execute(object parameter)
  {
    MessageBox.Show((parameter ?? "null").ToString());
  }
  public bool CanExecute(object parameter)
  {
    return true;
  }
  public event EventHandler CanExecuteChanged;
}

<Button
  Content="Test"
  Tag="Foo"
  Padding="5"
  VerticalAlignment="Center"
  HorizontalAlignment="Center"
  Command="{Binding}"
  CommandParameter="{Binding Tag, RelativeSource={RelativeSource Self}}" />

{Binding Tag, RelativeSource={RelativeSource Self}}