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

// Modifies the text graphic of a selected annotation feature in the specified annotation layer.
await QueuedTask.Run(() =>
{
  var selection = annotationLayer.GetSelection();
  if (selection.GetCount() == 0)
    return;
  // use the first selected feature 
  var insp = new Inspector();
  insp.Load(annotationLayer, 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
  {
    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
  }
});

//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...
public class ProSnippetAnnotationConstructionTool : MapTool
{
  public ProSnippetAnnotationConstructionTool()
  {
    IsSketchTool = true;
    UseSnapping = true;
    SketchType = SketchGeometryType.Point;
  }

  /// <summary>
  /// Handles the completion of a sketch operation and creates an annotation feature based on the provided geometry.
  /// </summary>
  protected async override Task<bool> OnSketchCompleteAsync(Geometry geometry)
  {
    if (CurrentTemplate == null || geometry == null)
      return false;
    // Create an edit operation
    var createOperation = new EditOperation
    {
      Name = string.Format("Create {0}", CurrentTemplate.Layer.Name),
      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 = 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;
  }
}