DialogWindow.xaml

<controls:MetroWindow x:Class="Mahapps.Metro.Tests.DialogWindow"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:controls="http://metro.mahapps.com/winfx/xaml/controls"
             xmlns:dialogs="clr-namespace:MahApps.Metro.Controls.Dialogs;assembly=MahApps.Metro"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300">
    <controls:MetroWindow.Resources>
        <dialogs:CustomDialog x:Key="CustomDialog">
            <TextBlock x:Name="TheDialogBody" Text="{Binding Path=Text}" />
        </dialogs:CustomDialog>
    </controls:MetroWindow.Resources>
</controls:MetroWindow>