public int VcsWkid {get;}

{
  // Use a builder convenience method or use a builder constructor.

  // custom VCS - use vertical shift of -1.23 instead of 0
  string custom_vWkt = @"VERTCS[""SHD_height"",VDATUM[""Singapore_Height_Datum""],PARAMETER[""Vertical_Shift"",-1.23],PARAMETER[""Direction"",-1.0],UNIT[""Meter"",1.0]]";

  // Builder convenience methods don't need to run on the MCT.
  SpatialReference sr4326_customVertical = SpatialReferenceBuilder.CreateSpatialReference(4326, custom_vWkt);
  // SpatialReferenceBuilder properties
  //   sr4326_customVertical.wkid == 4326
  //   sr4326_customVertical.vert_wkid == 0
  //   sr4326_customVertical.vert_wkt == custom_vWkt
  //   sr4326_customVertical.hasVcs == true

  // SpatialReferenceBuilder constructors need to run on the MCT.
  ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (SpatialReferenceBuilder sb = new SpatialReferenceBuilder(4326, custom_vWkt))
    {
      // do something with the builder

      sr4326_customVertical = sb.ToSpatialReference();
    }
  });
}