StTest/MyTests/RegAuth.xaml

<Window x:Class="MyTests.RegAuth"
        ResizeMode="NoResize" WindowStyle="SingleBorderWindow"
        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:MyTests"
        mc:Ignorable="d"
        Title="RegAuth" Height="475" Width="475">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50*"/>
            <RowDefinition Height="25*"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center">
            <Button Content="Регистрация" Width="140" Height="75" FontSize="20" Margin="40, 0, 40, 0" Click="WindowRegistration"></Button>
            <Button Content="Вход"  Width="140" Height="75" FontSize="25" Margin="40, 0, 40, 0" Click="WindowEntry"></Button>
        </StackPanel>
        <Frame
            Name="MainFrame"
            NavigationUIVisibility="Hidden"
            Grid.Row="0">
        </Frame>
    </Grid>
</Window>