Custom TextBox not updating property when change

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}" />