testexam/exam/AuthWindow.axaml

<Window xmlns="https://github.com/avaloniaui"
        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"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="exam.AuthWindow"
        Title="Авторизация">
        <Grid>
                <Grid.RowDefinitions>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                </Grid.RowDefinitions>
                
                <TextBox x:Name="LoginTextBox" Watermark="Логин" Margin="10, 20"
                         Grid.Row="0"/>
                <TextBox x:Name="PasswordTextBox" Watermark="Пароль" PasswordChar="*" Margin="10,20"
                         Grid.Row="1"/>
                <TextBlock x:Name="ErrorTextBlock" IsVisible="False" Foreground="Red" FontSize="14"
                           Margin="10,20"
                           Grid.Row="2"/>
                <Button x:Name="AuthButton" Click="AuthButton_OnClick" Content="Войти" Margin="10,20"
                        Grid.Row="3"/>
                <Button x:Name="RegButton" Click="RegButton_OnClick" Content="Зарегистрироваться" Margin="10,20"
                        Grid.Row="4"/>
        </Grid>
</Window>