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.
    }
  }
}