public class FeatureDatasetDescription : Description 

public void CreateAnnotationFeatureClassUsingExistingAnnotationFeatureClassInDataset(Geodatabase geodatabase)
{
    // Create a Cities annotation feature class inside Places feature dataset using existing annotation feature class 

    // Feature dataset name
    string featureDatasetName = "Places";

    // Annotation feature class name
    string annotationFeatureClassName = "CitiesAnnotation";

    // Create a SchemaBuilder object
    SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

    // Open existing annotation feature class name
    using (AnnotationFeatureClass existingAnnotationFeatureClass = geodatabase.OpenDataset<AnnotationFeatureClass>("ExistingAnnotationFeatureClass"))
    {

        // Create Feature dataset description
        FeatureDatasetDescription featureDatasetDescription =
          new FeatureDatasetDescription(featureDatasetName, existingAnnotationFeatureClass.GetDefinition().GetSpatialReference());

        // Add the creation of the Places dataset to DDL task
        FeatureDatasetToken featureDatasetToken = schemaBuilder.Create(featureDatasetDescription);

        // Create an annotation feature class description using existing annotation feature class
        AnnotationFeatureClassDescription annotationFeatureClassDescription = new AnnotationFeatureClassDescription(annotationFeatureClassName, existingAnnotationFeatureClass.GetDefinition())
        {
            IsAutoCreate = true,
            IsSymbolIDRequired = false,
            IsUpdatedOnShapeChange = true
        };

        // Add the creation of the Cities annotation feature class inside Places feature dataset
        schemaBuilder.Create(new FeatureDatasetDescription(featureDatasetToken), annotationFeatureClassDescription);

        // Execute the DDL
        bool success = schemaBuilder.Build();

        // Inspect error messages
        if (!success)
        {
            IReadOnlyList<string> errorMessages = schemaBuilder.ErrorMessages;
            //etc.
        }
    }
}