public Geometry GetDrawingOutline( 
   long oid,
   MapView view,
   DrawingOutlineType outlineType
)

{
    if (featureLayer == null)
        return;

    var mv = MapView.Active;
    //Note: Run within a QueuedTask
    using (var table = featureLayer.GetTable())
    {
        using (var rc = table.Search())
        {
            //get the first feature...
            //...assuming at least one feature gets retrieved
            rc.MoveNext();
            var oid = rc.Current.GetObjectID();

            //Use DrawingOutlineType.BoundingEnvelope to retrieve a generalized
            //mask geometry or "Box". The mask will be in the same SpatRef as the map
            var mask_geom = featureLayer.GetDrawingOutline(oid, mv, DrawingOutlineType.Exact);

            //TODO - use the mask geometry...
        }
    }
}

{
  var mv = MapView.Active;

  // Note: QueuedTask is required to access the AnnotationFeature
  {
    //get the first annotation feature...
    //...assuming at least one feature gets selected
    using var fc = annotationLayer.GetFeatureClass();
    using var rc = fc.Search();
    rc.MoveNext();
    using var row = rc.Current;
    var oid = row.GetObjectID();

    //Use DrawingOutlineType.BoundingEnvelope to retrieve a generalized
    //mask geometry or "Box". The mask will be in the same SpatRef as the map.
    //The mask will be constructed using the anno class reference scale
    var mask_geom = annotationLayer.GetDrawingOutline(oid, mv, DrawingOutlineType.Exact);
  }
}