How to set TabItem's header from code-behind WPF

Question

I have a menu with RadioButton that opens a new tab when checked, now when I create the new tab I'm trying to add a style for the header in App.XAML but when I try to add an event to the (x) to close the tab it takes me to App.XAML.CS which I don't want to, because on that page I can't reach the TabControl that exists in my MainWindow So I'm trying to add the style from Code-Behind but I didn't find out how to do it. This is my style

<Border
       x:Name="PART_Border"
       Background="{TemplateBinding Background}"
       BorderThickness="1"
       BorderBrush="{x:Null}"
       Margin="5,-3,3,4"
       CornerRadius="5">
       <DockPanel>
           <Button Name="btnDelete"
                   DockPanel.Dock="Right"
                   Margin="5 0 0 0"
                   Padding="0"
                   Click="BtnCloseTab_Click"
                   Style="{DynamicResource TabCloseBtn}">
                   <Image Source="img\White\closeWhite.png" Height="11" Width="11"/>
           </Button>
           <TextBlock Text="{Binding RelativeSource={RelativeSource AncestorType=TabItem }, Path=Header}" />
       </DockPanel>
</Border>

When I go to the event here it takes me to App.XAML.CS but I want to move to MainWindow.XAML.CS I already search a lot about it but nothing was helpful for my situation.