Hi,
You have to edit the control template of AutoCompleteBox to acheive this.
<ControlTemplate TargetType="sdk:AutoCompleteBox">
<Grid Opacity="{TemplateBinding Opacity}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="PopupStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0:0:0.1" To="PopupOpened"/>
<VisualTransition GeneratedDuration="0:0:0.2" To="PopupClosed"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="PopupOpened">
<Storyboard>
<DoubleAnimation To="1.0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="PopupBorder"/>
</Storyboard>
</VisualState>
<VisualState x:Name="PopupClosed">
<Storyboard>
<DoubleAnimation To="0.0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="PopupBorder"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="ValidationStates">
<VisualState x:Name="Valid"/>
<VisualState x:Name="InvalidUnfocused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="ValidationErrorElement">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="InvalidFocused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="ValidationErrorElement">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="IsOpen" Storyboard.TargetName="validationTooltip">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<System:Boolean>True</System:Boolean>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<TextBox x:Name="Text" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Foreground="{TemplateBinding Foreground}" IsTabStop="True" Margin="0" Padding="{TemplateBinding Padding}" Style="{TemplateBinding TextBoxStyle}"/>
<Border x:Name="ValidationErrorElement" BorderBrush="#FFDB000C" BorderThickness="1" CornerRadius="1" Visibility="Collapsed">
<ToolTipService.ToolTip>
<ToolTip x:Name="validationTooltip" DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}}" Placement="Right" PlacementTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Template="{StaticResource CommonValidationToolTipTemplate}">
<ToolTip.Triggers>
<EventTrigger RoutedEvent="Canvas.Loaded">
<BeginStoryboard>
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="IsHitTestVisible" Storyboard.TargetName="validationTooltip">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<System:Boolean>true</System:Boolean>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</ToolTip.Triggers>
</ToolTip>
</ToolTipService.ToolTip>
<Grid Background="Transparent" HorizontalAlignment="Right" Height="12" Margin="1,-4,-4,0" VerticalAlignment="Top" Width="12">
<Path Data="M 1,0 L6,0 A 2,2 90 0 1 8,2 L8,7 z" Fill="#FFDC000C" Margin="1,3,0,0"/>
<Path Data="M 0,0 L2,0 L 8,6 L8,8" Fill="#ffffff" Margin="1,3,0,0"/>
</Grid>
</Border>
<Popup x:Name="Popup">
<Grid Opacity="{TemplateBinding Opacity}">
<Border x:Name="PopupBorder" BorderThickness="0" Background="#11000000" HorizontalAlignment="Stretch" Opacity="0">
<Border.RenderTransform>
<TranslateTransform X="1" Y="1"/>
</Border.RenderTransform>
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="0" HorizontalAlignment="Stretch" Opacity="1.0" Padding="0">
<Border.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FFDDDDDD" Offset="0"/>
<GradientStop Color="#AADDDDDD" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Border.RenderTransform>
<TransformGroup>
<TranslateTransform X="-1" Y="-1"/>
</TransformGroup>
</Border.RenderTransform>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Button Grid.Row="0"/> <!-- Here is have added the Button... you can add the desired functionality-->
<ListBox Grid.Row="1" x:Name="Selector" BorderThickness="0" Background="{TemplateBinding Background}" Foreground="{TemplateBinding Foreground}" ScrollViewer.HorizontalScrollBarVisibility="Auto" ItemTemplate="{TemplateBinding ItemTemplate}" ItemContainerStyle="{TemplateBinding ItemContainerStyle}" ScrollViewer.VerticalScrollBarVisibility="Auto"/>
</Grid>
</Border>
</Border>
</Grid>
</Popup>
</Grid>
</ControlTemplate>
