gistfile1.cs

public partial class LoadingDialog : BaseMetroDialog
{
    public static readonly DependencyProperty MessageProperty = DependencyProperty.Register("Message", typeof(string), typeof(MessageDialog), new PropertyMetadata(default(string)));

    internal LoadingDialog(MetroWindow parentWindow)
        : this(parentWindow, null)
    {
    }

    internal LoadingDialog(MetroWindow parentWindow, MetroDialogSettings settings)
        : base(parentWindow, settings)
    {
        InitializeComponent();
    }

    public string Message
    {
        get { return (string)GetValue(MessageProperty); }
        set { SetValue(MessageProperty, value); }
    }
}

gistfile2.xml

<Dialogs:BaseMetroDialog x:Class="McKinsey.MarketSimulator.Controls.LoadingDialog"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:Controls="clr-namespace:MahApps.Metro.Controls.Dialogs;assembly=MahApps.Metro"
    xmlns:Dialogs="clr-namespace:MahApps.Metro.Controls.Dialogs;assembly=MahApps.Metro"
    xmlns:local="clr-namespace:McKinsey.MarketSimulator.Controls"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
    mc:Ignorable="d" 
    d:DesignHeight="300" d:DesignWidth="300">
    <Dialogs:BaseMetroDialog.DialogBody>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="*" />
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>
            <TextBlock Margin="0 5 0 0"
                       FontSize="15"
                       Text="{Binding Message, RelativeSource={RelativeSource AncestorType=local:LoadingDialog, Mode=FindAncestor}, UpdateSourceTrigger=PropertyChanged}"
                       TextWrapping="Wrap"
                       Grid.Row="0"
                       Foreground="{Binding Foreground, RelativeSource={RelativeSource AncestorType=local:LoadingDialog, Mode=FindAncestor}, UpdateSourceTrigger=PropertyChanged}" />
        </Grid>
    </Dialogs:BaseMetroDialog.DialogBody>
</Dialogs:BaseMetroDialog>