public int AdditionalAngle {get; set;}

{
  // Note: call within QueuedTask.Run()
  {
    //Check if the label engine is Maplex or standard.
    CIMGeneralPlacementProperties labelEngine = MapView.Active.Map.GetDefinition().GeneralPlacementProperties;
    if (labelEngine is CIMStandardGeneralPlacementProperties)
      return;

    //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();
    //Modify label Rotation
    CIMMaplexRotationProperties rotationProperties = new CIMMaplexRotationProperties
    {
      Enable = true, //Enable rotation
      RotationField = "ELEVATION", //Field that is used to define rotation angle
      AdditionalAngle = 15, //Additional rotation 
      RotationType = MaplexLabelRotationType.Arithmetic,
      AlignmentType = MaplexRotationAlignmentType.Perpendicular,
      AlignLabelToAngle = true
    };
    theLabelClass.MaplexLabelPlacementProperties.RotationProperties = rotationProperties;
    lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
    featureLayer.SetDefinition(lyrDefn); //set the layer's definition
  }
}