I am setting focus on a Textbox like this:

<DockPanel
        Margin="0,0,0,0"
        LastChildFill="True"
        FocusManager.FocusedElement="{Binding ElementName=messengerTextToSend}">
        <ListBox
            x:Name="messengerLabelParticipants"
            DockPanel.Dock="Top" Height="79" Margin="0,1,0,0" Padding="0"
            Background="{x:Null}" BorderBrush="{x:Null}" BorderThickness="0"
            AllowDrop="True"
            ItemsSource="{Binding Path=involvedUsers}" ItemTemplate="{StaticResource chatParticipants}" Tag="{Binding Path=chatSessionID}"
            Drop="participantList_Drop" DragEnter="participantList_DragEnter" DragLeave="messengerLabelParticipants_DragLeave">
        </ListBox>
        <TextBox
            x:Name="messengerTextToSend"
            Focusable="True"
            Margin="10,0,10,10"
            DockPanel.Dock="Bottom" Height="100"
            Tag="{Binding Path=.}"
            KeyUp="messengerTextToSend_KeyUp"
            Cursor="IBeam"
            Style="{StaticResource messengerTextBoxSendText}"/>
        <ScrollViewer 
            x:Name="messengerScroller" 
            Template="{DynamicResource ScrollViewerControlTemplate1}" 
            ScrollChanged="messengerScroller_ScrollChanged" Loaded="messengerScroller_Loaded"
            Margin="0,10,0,10">
            <ListBox
                x:Name="messengerListMessages"
                Margin="10,0,0,0" Padding="0" 
                Background="{x:Null}" BorderBrush="{x:Null}" BorderThickness="0"
                IsSynchronizedWithCurrentItem="True"
                ItemsSource="{Binding Path=messages}" ItemTemplateSelector="{StaticResource messageTemplateSelector}">
            </ListBox>
        </ScrollViewer>
    </DockPanel>

However, when the page load, although the Textbox visually appears to have focus, the cursor is static and I have to manually either click on the Textbox or tab to it in order to start typing. I'm not sure what I'm doing wrong but I've tried every setting, inclduing setting it in the code to get it working. Any assistance would be greatly appreciated.