AutorizationGrigorenko/MainWindow.xaml

<Window x:Class="AutorizationGrrigorenko.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:AutorizationGrrigorenko"
        mc:Ignorable="d"
        Title="Авторизация" Height="450" Width="800">
    <Grid>
        <Grid.ColumnDefinitions>

        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>

            <RowDefinition Height="42"/>
            <RowDefinition />
            <RowDefinition Height="42"/>

        </Grid.RowDefinitions>
        <Rectangle Fill="#FFF4F4F5" HorizontalAlignment="Left" Height="42" Grid.Row="2" Stroke="Black" VerticalAlignment="Top" Width="792" StrokeThickness="0"/>
        <Button x:Name="ButtonBack" Click="ButtonBack_Click" Content="Назад" HorizontalAlignment="Left" Margin="10,12,0,0" VerticalAlignment="Top" Width="75"/>
        <Frame Content="MainFrame" Grid.Row="1" x:Name="MainFrame" Source="Pages/AuthPage.xaml" Navigated="MainFrame_Navigated" />

    </Grid>
</Window>