protjekt/WpfApp5/MainWindow.xaml

<Window x:Class="WpfApp5.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp5"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid Height="419" VerticalAlignment="Top">
        <Grid.RowDefinitions>
            <RowDefinition Height="42"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="42"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="295*"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Grid HorizontalAlignment="Left" Height="100" Margin="50,16.562,0,0" VerticalAlignment="Top" Width="100" Grid.Column="1" Grid.Row="1"/>
        <Rectangle StrokeThickness="0" Fill="#FFF4F4F5" HorizontalAlignment="Left" Grid.Row="2" Stroke="Black" Width="792" Grid.ColumnSpan="3"/>
        <Button x:Name="ButtonBack" Content="Назад" Grid.Column="1" HorizontalAlignment="Left" Margin="10,10,0,10" VerticalAlignment="Top" Width="75" Click="Button_Click"/>
        <Frame x:Name="MainFrame" Content="Frame" Source="Pages/AuthPage.xaml" Grid.Column="1" HorizontalAlignment="Left" Height="335" Grid.Row="1" Navigated="MainFrame_OnNavigated" VerticalAlignment="Top" Width="790" Grid.ColumnSpan="2" Margin="0,0,-0.333,0"/>
        <Label Content="Label"  Grid.Column="1" HorizontalAlignment="Left" Margin="127,178,0,0" Grid.Row="1" VerticalAlignment="Top"/>

    </Grid>
</Window>