bonjour/WpfApp1/MainWindow.xaml

<Window x:Class="WpfApp1.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:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid Margin="0,29,0,0">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100"/>
            <ColumnDefinition Width="235*"/>
            <ColumnDefinition Width="100"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="40"/>
            <RowDefinition/>
            <RowDefinition Height="42"/>
        </Grid.RowDefinitions>
        <Rectangle Fill="#FFF4F4F5" Grid.Row="2" StrokeThickness="0"/>
        <Button x:Name="ButtonBack" Click="Button_Click" Content="Назад" HorizontalAlignment="Left" Margin="10,10,0,10" Width="75"/>
        <Frame x:Name="MainFrame" Source="Pages/AuthPage.xaml" Grid.Row="1" Grid.ColumnSpan="3" Navigated="MainFrame_OnNavigated"/>
    </Grid>
</Window>