<UserControl
         xmlns:Dialog="clr-namespace:MahApps.Metro.Controls.Dialogs;assembly=MahApps.Metro"
        Dialog:DialogParticipation.Register="{Binding}">

viewModel.Initialize(DialogCoordinator.Instance);

IDialogCoordinator _DialogCoordinator;  // set with viewModel.Initialize() called from the view

private async Task _SomeCmdExecute()
    {
      await _DialogCoordinator.ShowMessageAsync(this, "HEADER", "TEST");
    }