public string RotationField {get; set;}

//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();
//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