public CIMColor Color {get; set;}

await QueuedTask.Run(() =>
  {
    // annotationLayer is ~your~ Annotation layer...
    // annotationPoint is ~your~ Annotation's location geometry ...
    var op = new EditOperation();
    // Use the inspector
    var insp = new Inspector();
    insp.LoadSchema(annotationLayer);
    // get the annotation properties from the inspector
    AnnotationProperties annoProperties = insp.GetAnnotationProperties();
    // change the annotation text 
    annoProperties.TextString = DateTime.Now.ToLongTimeString();
    // change font color to green
    annoProperties.Color = ColorFactory.Instance.GreenRGB;
    // change the horizontal alignment
    annoProperties.HorizontalAlignment = HorizontalAlignment.Center;
    annoProperties.Shape = annotationPoint;
    // set the annotation properties back on the inspector
    insp.SetAnnotationProperties(annoProperties);
    // create the annotation
    op.Create(annotationLayer, insp);
    if (!op.IsEmpty)
    {
      var result = op.Execute(); //Execute and ExecuteAsync will return true if the operation was successful and false if not
    }
  });