public string VcsWkt {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);
// sr4326_customVertical.wkid = 4326
// sr4326_customVertical.vert_wkid = 0
// sr4326_customVertical.vert_wkt = custom_vWkt
// sr4326_customVertical.hasVcs = true

// Builder 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();
  }
});