Custom TextBox not updating property when change - wpf

Question

I have a custom TextBox control where the defaul value is the Tag string. When the TextBox is empty it shows a TextBlock on top with the Tag string with other color and centered. The control works on almost every scenario but I am having issues with one specifically. I have a TexBox that need to update a property when I type so I thought that adding the UpdateSourceTrigger=PropertyChanged will update the peroperty, but it doesn't work when I use the custom style, it works only on default so I thought that the issue is on the custom Style, but I am too new on xaml to figure out. Could someone please enlight me on what I am doing wrong?
<TextBox Grid.Column="2" Margin="5" Style="{StaticResource DefaultTextBox}" Tag="Sesión" Text="{Binding Sesion, UpdateSourceTrigger=PropertyChanged}" Width="100"/>
<!--Directorio TextBox-->
<Style x:Key="DefaultTextBox" TargetType="TextBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Border Background="#ffffff" BorderBrush="#ffacacac" BorderThickness="1" CornerRadius="0">
<Grid>
<TextBox x:Name="TextBoxPersonalizado" Background="Transparent" BorderThickness="0" VerticalContentAlignment="Center" Text="{TemplateBinding Text}"/>
<TextBlock Foreground="#828282" HorizontalAlignment="Center" IsHitTestVisible="False" Text="{TemplateBinding Tag}" VerticalAlignment="Center">
<TextBlock.Style>
<Style TargetType="{x:Type TextBlock}">
<Style.Triggers>
<DataTrigger Binding="{Binding Text, ElementName=TextBoxPersonalizado}" Value="">
<Setter Property="Visibility" Value="Visible"/>
</DataTrigger>
</Style.Triggers>
<Setter Property="Visibility" Value="Hidden"/>
</Style>
</TextBlock.Style>
</TextBlock>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Answer 1

TemplateBinding is for one direction only.
That beeing said, you need to use RelativeSource inside your template.
Text="{Binding Text, RelativeSource={RelativeSource TemplatedParent}, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}"
Then you can use your customized TextBox like this.
<TextBox
x:Name="InputTextBox"
Grid.Column="0"
Width="100"
Margin="5"
Style="{StaticResource DefaultTextBox}"
Tag="Sesión" />
<TextBox Grid.Column="1" Text="{Binding ElementName=InputTextBox, Path=Text}" />