var messageBox = MessageBoxManager.GetMessageBoxCustom(new MessageBoxCustomParams
    {
        ContentTitle = "Custom Dialog",
        ContentMessage = "This is a dialog with multiple buttons.",
        ButtonDefinitions = new[]
        {
            new ButtonDefinition { Name = "OK", IsDefault = true },
            new ButtonDefinition { Name = "Cancel" },
            new ButtonDefinition { Name = "Ignore" }
        },
        WindowStartupLocation = WindowStartupLocation.CenterOwner
    });