How to set header for first tab?

            <sf:DocumentContainer.HeaderTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}"
                                                   FontWeight="{Binding Modified,
                                                    Converter={StaticResource BoolToFontWeight}}"
                                         />
                </DataTemplate>
            </sf:DocumentContainer.HeaderTemplate>

And it works fine. But I want the first tab to show static header "Start". I thought it would be simple. Just add item into container and set sf:DocumentContainer.Header property. But it doesn't work. I tried also some datatriggers, but I failed. How to do it?

tabControl = VisualUtils.FindDescendant(sender as Visual, typeof(DocumentTabControl)) as DocumentTabControl;

//find template and add it to first item and add it to first item:

DataTemplate template = TryFindResource(templateKeyName) as DataTemplate;

TabItemExt item = tabControl.Items[0] as TabItemExt;

if(item != null)

  item.HeaderTemplate = template;