public string DefaultExt {get; set;}

{
  SaveItemDialog saveMapFileDialog = new()
  {
    Title = "Save Map File",
    InitialLocation = @"C:\Data\NewYork\Counties\Maps",
    DefaultExt = @"mapx",
    Filter = ItemFilters.Maps_All,
    OverwritePrompt = true
  };
  bool? result = saveMapFileDialog.ShowDialog();

  if (result == true)
  {
    ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show("Returned file name: " + saveMapFileDialog.FilePath);
  }
}