public CIMSymbolReference TextSymbol {get; set;}

public async Task CreatingAnAnnotationFeature(Geodatabase geodatabase)
{
    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
        using (AnnotationFeatureClass annotationFeatureClass = geodatabase.OpenDataset<AnnotationFeatureClass>("Annotation // feature // class // name"))
        using (AnnotationFeatureClassDefinition annotationFeatureClassDefinition = annotationFeatureClass.GetDefinition())
        using (RowBuffer rowBuffer = annotationFeatureClass.CreateRowBuffer())
        using (AnnotationFeature annotationFeature = annotationFeatureClass.CreateRow(rowBuffer))
        {
            annotationFeature.SetAnnotationClassID(0);
            annotationFeature.SetStatus(AnnotationStatus.Placed);

            // Get the annotation labels from the label collection
            IReadOnlyList<CIMLabelClass> labelClasses =
        annotationFeatureClassDefinition.GetLabelClassCollection();

            // Setup the symbol reference with the symbol id and the text symbol
            CIMSymbolReference cimSymbolReference = new CIMSymbolReference();
            cimSymbolReference.Symbol = labelClasses[0].TextSymbol.Symbol;
            cimSymbolReference.SymbolName = labelClasses[0].TextSymbol.SymbolName;

            // Setup the text graphic
            CIMTextGraphic cimTextGraphic = new CIMTextGraphic();
            cimTextGraphic.Text = "Charlotte, North Carolina";
            cimTextGraphic.Shape = new MapPointBuilderEx(new Coordinate2D(-80.843, 35.234), SpatialReferences.WGS84).ToGeometry();
            cimTextGraphic.Symbol = cimSymbolReference;

            // Set the symbol reference on the graphic and store
            annotationFeature.SetGraphic(cimTextGraphic);
            annotationFeature.Store();
        }
    });
}

//Note: call within QueuedTask.Run()

//Get the layer's definition
var lyrDefn = featureLayer.GetDefinition() as CIMFeatureLayer;
//Get the label classes - we need the first one
var listLabelClasses = lyrDefn.LabelClasses.ToList();
var theLabelClass = listLabelClasses.FirstOrDefault();
//Set the label classes' symbol to the custom text symbol
//Refer to the ProSnippets-TextSymbols wiki page for help with creating custom text symbols.
//Example: var textSymbol = await CreateTextSymbolWithHaloAsync();
theLabelClass.TextSymbol.Symbol = textSymbol;
lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition
                                     //set the label's visiblity
featureLayer.SetLabelVisibility(true);