Need help with Attached Command (Behavior) that is not working...

I've googled, read and re-googled and re-read and I'm sorry but I'm starting to see cross-eyed on this particular issue...  I've tried about everything I can find/see on the net and nothing seems to get me a bindable 'command' attached property.  I'm apparently missing something fundamental and just need help figuring out what that is.

Here's the code...

SpecialEnterKeyHandling.cs:

namespace MyLib.Shared.UIExtensions{
   public static class SpecialEnterKeyHandling   {
      #region Attached Enabled Property
      public static bool GetEnabled(DependencyObject obj)
      {
         return (bool)obj.GetValue(EnabledProperty);
      }
      public static void SetEnabled(DependencyObject obj, bool value)
      {
         obj.SetValue(EnabledProperty, value);
      }
      public static readonly DependencyProperty EnabledProperty =
          DependencyProperty.RegisterAttached("Enabled", typeof(bool), typeof(SpecialEnterKeyHandling),
                                             new FrameworkPropertyMetadata(OnEnabledPropertyChanged));
      static void OnEnabledPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
      {
         var element = d as UIElement;
         if (element == null) return;
         if ((bool)e.NewValue) element.KeyDown += EnterKeyDownHandler;
         else element.KeyDown -= EnterKeyDownHandler;
         if ((element as DataGrid) != null)
         {
            if ((bool)e.NewValue) element.PreviewKeyDown += EnterKeyDownHandler;
            else element.PreviewKeyDown -= EnterKeyDownHandler;
         } // End if
      }
      #endregion
      #region Attached Command Property
      public static ICommand GetCommandProperty(DependencyObject obj)
      {
         return (ICommand)obj.GetValue(CommandProperty);
      }
      public static void SetCommandProperty(DependencyObject obj, ICommand value)
      {
         obj.SetValue(CommandProperty, value);
      }
      public static readonly DependencyProperty CommandProperty =
          DependencyProperty.RegisterAttached("Command", typeof(ICommand), typeof(SpecialEnterKeyHandling),
                                             new FrameworkPropertyMetadata(OnCommandPropertyChanged));
      static void OnCommandPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
      {
         // what to do here? as far as I can tell this wouldn't be needed, but....
      }
      #endregion
      #region Special Enter Key Handling
      static void EnterKeyDownHandler(object sender, KeyEventArgs e)
      {
         var element = sender as UIElement;
         if (element == null) return;
         if (!e.Key.Equals(Key.Enter)) return;
         if (element.GetValue(SpecialEnterKeyHandling.CommandProperty) != null)
         {
            ICommand Command = (element.GetValue(SpecialEnterKeyHandling.CommandProperty) as ICommand);
            if (Command.CanExecute(sender))
            {
               Command.Execute(sender);
            } // End if
         }
         else
         {
            element.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
         } // End if
         e.Handled = true;
      }
      #endregion
   }
}

MyScreen.Xaml:

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:MyLib.Dialogs" x:Class="SearchDialog"
        xmlns:UIExt="clr-namespace:MyLib.Shared.UIExtensions;assembly=MyLib.Shared"
        Title="Account Search" Height="500" Width="700" BorderBrush="Black" ResizeMode="CanResizeWithGrip">
    <Grid FocusManager.FocusedElement="{Binding ElementName=AccountTxt}">
        <DockPanel Name="TopPanel" Height="50" Background="RoyalBlue" VerticalAlignment="Top" Margin="0,0,0,0">
            <Grid HorizontalAlignment="Left" VerticalAlignment="Top" Margin="1">
                <Label Content="Account" HorizontalAlignment="Left" VerticalAlignment="Top" Style="{DynamicResource LabelBlack}" />
                <TextBox x:Name="AccountTxt" HorizontalAlignment="Left" VerticalAlignment="Top" Width="52.8" Margin="0,25,0,0" Text="{Binding Account}" UIExt:SpecialEnterKeyHandling.Enabled="True" UIExt:SpecialEnterKeyHandling.CommandProperty="{Binding SearchCommand}" />
            </Grid>
</DockPanel>
    </Grid>
</Window>

First thing that strikes me as extremely odd...  The Enabled attached property appears as I would expect it...  The Command attached command(behavior) appears as CommandProperty (shouldn't it be Command and not CommandProperty?)

And then finally.  No matter what I do I get:

Error7A 'Binding' cannot be used within a 'TextBox' collection. A 'Binding' can only be set on a DependencyProperty of a DependencyObject. SearchDialog.xaml23198

Again, I have tried adjusting other's examples, tweaking my code above in so many ways now I can't remember them all, still none seem to over come the above problems... Can someone please help me understand what I am doing wrong.  I really need to add Command to an a TextBox (and Label) which appear to not have this property and bind it my ViewModel Command (SearchCommand).

George P Botuwell, Programmer