{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=PasswordBox}}

<Label Margin="15" Padding="15">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>

        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Grid Grid.Row="0" Grid.ColumnSpan="2">
            <Grid.RowDefinitions>
                <RowDefinition />
                <RowDefinition />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition />
                <ColumnDefinition />
            </Grid.ColumnDefinitions>
            <Label Style="{StaticResource InfoLabel}" Content="My Super Logotype" Grid.RowSpan="2" Grid.Column="0"
                   Grid.ColumnSpan="2" FontSize="32"
                   VerticalAlignment="Center"
                   HorizontalAlignment="Center" />
            <Grid Grid.Column="1" Grid.Row="0">
                <Label Content="SubTAG" Style="{StaticResource InfoLabel}" VerticalAlignment="Bottom"
                       HorizontalAlignment="Left"
                       Margin="47,-47,-13,13" />
            </Grid>
        </Grid>
        <Grid Grid.Column="0" Grid.ColumnSpan="2" Grid.Row="1">
            <Grid.RowDefinitions>
                <RowDefinition Height="*" />
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>

            <Label Grid.Column="0">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="Auto" />
                        <ColumnDefinition Width="*" />
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition />
                        <RowDefinition />
                    </Grid.RowDefinitions>
                    <Label Margin="0,2" VerticalAlignment="Center" HorizontalAlignment="Right" Content="Логин:" />
                    <TextBox Text="{Binding UserName}" Grid.Row="0" Grid.Column="1"
                             HorizontalContentAlignment="Stretch"
                             VerticalAlignment="Center" FontSize="14"
                             HorizontalAlignment="Left" Width="145" />
                    <Label Grid.Column="0" Grid.Row="1" Content="Пароль:" />
                    <PasswordBox x:Name="UserPassword" Grid.Column="1" Width="145" Grid.Row="1" FontSize="14"
                                 HorizontalAlignment="Stretch" />
                </Grid>
            </Label>

            <Grid Grid.Row="1">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>
                <Button Command="{Binding AuthUserCommand}"
                        CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=PasswordBox}}"
                        Margin="0,0,7,0"
                        HorizontalAlignment="Stretch" Style="{StaticResource SuccessBtn}"
                        Content="Войти" />
                <Button Margin="0,0,7,0" MinWidth="12" HorizontalAlignment="Right" Grid.Column="1"
                        Style="{StaticResource WarningBtn}"
                        Content="Отмена" Command="{Binding Close}" />
            </Grid>
        </Grid>
    </Grid>
</Label>