public enum HorizontalAlignment : System.Enum, System.IComparable, System.IConvertible, System.IFormattable  

//In your config.daml...set the categoryRefID
//<tool id="..." categoryRefID="esri_editing_construction_annotation" caption="Create Anno" ...>

//Sketch type Point or Line or BezierLine in the constructor...
//internal class AnnoConstructionTool : MapTool  {
//  public AnnoConstructionTool()  {
//    IsSketchTool = true;
//    UseSnapping = true;
//    SketchType = SketchGeometryType.Point;
//

protected async override Task<bool> OnSketchCompleteAsync(Geometry geometry)
{
  if (CurrentTemplate == null || geometry == null)
    return false;

  // Create an edit operation
  var createOperation = new EditOperation();
  createOperation.Name = string.Format("Create {0}", CurrentTemplate.Layer.Name);
  createOperation.SelectNewFeatures = true;

  var insp = CurrentTemplate.Inspector;
  var result = await QueuedTask.Run(() =>
  {
    // get the annotation properties class
    AnnotationProperties annoProperties = insp.GetAnnotationProperties();
    // set custom annotation properties
    annoProperties.TextString = "my custom text";
    annoProperties.Color = ColorFactory.Instance.RedRGB;
    annoProperties.FontSize = 24;
    annoProperties.FontName = "Arial";
    annoProperties.HorizontalAlignment = ArcGIS.Core.CIM.HorizontalAlignment.Right;
    annoProperties.Shape = geometry;
    // assign annotation properties back to the inspector
    insp.SetAnnotationProperties(annoProperties);

    // Queue feature creation
    createOperation.Create(CurrentTemplate.Layer, insp);

    if (!createOperation.IsEmpty)
    {
      // Execute the operation
      return createOperation.Execute(); //Execute and ExecuteAsync will return true if the operation was successful and false if not
    }
    else
      return false;
  });
  return result;
}

await QueuedTask.Run(() =>
{

  var selection = annoLayer.GetSelection();
  if (selection.GetCount() == 0)
    return;

  // use the first selelcted feature 
  //at 2.x - var insp = new Inspector(true);
  var insp = new Inspector();
  insp.Load(annoLayer, selection.GetObjectIDs().FirstOrDefault());

  // getAnnoProperties should return null if not an annotation feature
  AnnotationProperties annoProperties = insp.GetAnnotationProperties();
  // get the textGraphic
  CIMTextGraphic textGraphic = annoProperties.TextGraphic;

  // change text
  textGraphic.Text = "Hello world";

  // set x,y offset via the symbol
  var symbol = textGraphic.Symbol.Symbol;
  var textSymbol = symbol as CIMTextSymbol;
  textSymbol.OffsetX = 2;
  textSymbol.OffsetY = 3;

  textSymbol.HorizontalAlignment = HorizontalAlignment.Center;

  // load the updated textGraphic
  annoProperties.LoadFromTextGraphic(textGraphic);
  // assign the annotation properties back 
  insp.SetAnnotationProperties(annoProperties);

  EditOperation op = new EditOperation();
  op.Name = "modify symbol";
  op.Modify(insp);
  if (!op.IsEmpty)
  {
    bool result = op.Execute(); //Execute and ExecuteAsync will return true if the operation was successful and false if not
  }
});