public Geometry GetGraphicOutline()

var annoLayer = MapView.Active.Map.GetLayersAsFlattenedList()
                               .OfType<AnnotationLayer>().FirstOrDefault();
if (annoLayer == null)
    return;

QueuedTask.Run(() =>
{
    //get the first annotation feature...
    //...assuming at least one feature gets selected
    using (var fc = annoLayer.GetFeatureClass())
    {
        using (var rc = fc.Search())
        {
            rc.MoveNext();
            using (var af = rc.Current as AnnotationFeature)
            {
                var outline_geom = af.GetGraphicOutline();
                //TODO - use the outline...

                //Note: 
                //var graphic = annoFeature.GetGraphic(); 
                //gets the CIMTextGraphic...
            }
        }
    }
});