MessagePopup.xaml

<local:ContentPopup x:Class="Telegram.Controls.MessagePopup"
                    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                    xmlns:local="using:Telegram.Controls"
                    mc:Ignorable="d">

    <StackPanel>
        <TextBlock x:Name="MessageLabel"
                   Style="{StaticResource BodyTextBlockStyle}" />
        <CheckBox x:Name="CheckBox"
                  Checked="CheckBox_Checked"
                  Unchecked="CheckBox_Checked"
                  Margin="0,16,0,-4"
                  Visibility="Collapsed" />
    </StackPanel>
</local:ContentPopup>