public CIMViewCamera Camera {get; set;}

public Task<Bookmark> AddBookmarkFromCameraAsync(Camera camera, string name)
{
  return QueuedTask.Run(() =>
  {
    //Set properties for Camera
    CIMViewCamera cimCamera = new CIMViewCamera()
    {
      X = camera.X,
      Y = camera.Y,
      Z = camera.Z,
      Scale = camera.Scale,
      Pitch = camera.Pitch,
      Heading = camera.Heading,
      Roll = camera.Roll
    };

    //Create new CIM bookmark and populate its properties
    var cimBookmark = new CIMBookmark() { Camera = cimCamera, Name = name, ThumbnailImagePath = "" };

    //Get the active map view.
    var mapView = MapView.Active;
    if (mapView == null)
      return null;

    //Add a new bookmark for the active map.
    return mapView.Map.AddBookmark(cimBookmark);
  });
}

public Task UpdateBookmarkExtentAsync(Bookmark bookmark, ArcGIS.Core.Geometry.Envelope envelope)
{
  return QueuedTask.Run(() =>
  {
    //Get the bookmark's definition
    var bookmarkDef = bookmark.GetDefinition();

    //Modify the bookmark's location
    bookmarkDef.Location = envelope;

    //Clear the camera as it is no longer valid.
    bookmarkDef.Camera = null;

    //Set the bookmark definition
    bookmark.SetDefinition(bookmarkDef);
  });
}