Singapore - EPSG:6917

WKT

HVCOORDSYS["HVC_SVY21_and_SHD_height", 
	GEOGCS["GCS_SVY21", 
		DATUM["D_SVY21", 
			SPHEROID["WGS_1984", 6378137.0, 298.257223563]], 
		PRIMEM["Greenwich", 0.0], 
		UNIT["Degree", 0.0174532925199433]], 
	VERTCS["SHD_height", 
		VDATUM["Singapore_Height_Datum"], 
		PARAMETER["Vertical_Shift", 0.0], 
		PARAMETER["Direction", 1.0], 
		UNIT["Meter", 1.0]]]

WKT2

COMPOUNDCRS["HVC_SVY21_and_SHD_height", 
	GEOGCRS["GCS_SVY21", 
		DATUM["D_SVY21", 
			ELLIPSOID["WGS_1984", 6378137.0, 298.257223563]], 
		PRIMEM["Greenwich", 0.0], 
		CS[ellipsoidal, 2], 
		AXIS["Latitude (lat)", north, 
			ORDER[1]], 
		AXIS["Longitude (lon)", east, 
			ORDER[2]], 
		ANGLEUNIT["Degree", 0.0174532925199433]], 
	VERTCRS["SHD_height", 
		VDATUM["Singapore_Height_Datum"], 
		CS[vertical, 1], 
		AXIS["Gravity-related height (H)", up, 
			LENGTHUNIT["Meter", 1.0]]], 
	ID["EPSG", "6917"]]

Data from @esri/proj-codes

{
  "wkid": 6917,
  "latestWkid": 6917,
  "pcsWkid": 0,
  "gcsWkid": 4757,
  "vcsWkid": 6916,
  "macro": "PE_HVC_SVY21_SHD_HT",
  "name": "HVC_SVY21_and_SHD_height",
  "wkt": "HVCOORDSYS[\"HVC_SVY21_and_SHD_height\",GEOGCS[\"GCS_SVY21\",DATUM[\"D_SVY21\",SPHEROID[\"WGS_1984\",6378137.0,298.257223563]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]],VERTCS[\"SHD_height\",VDATUM[\"Singapore_Height_Datum\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]]",
  "wkt2": "COMPOUNDCRS[\"HVC_SVY21_and_SHD_height\",GEOGCRS[\"GCS_SVY21\",DATUM[\"D_SVY21\",ELLIPSOID[\"WGS_1984\",6378137.0,298.257223563]],PRIMEM[\"Greenwich\",0.0],CS[ellipsoidal,2],AXIS[\"Latitude (lat)\",north,ORDER[1]],AXIS[\"Longitude (lon)\",east,ORDER[2]],ANGLEUNIT[\"Degree\",0.0174532925199433]],VERTCRS[\"SHD_height\",VDATUM[\"Singapore_Height_Datum\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]]],ID[\"EPSG\",\"6917\"]]",
  "description": "Singapore",
  "authority": "EPSG",
  "version": "8.5.1(10.8.0)",
  "deprecated": "no",
  "areaname": "Singapore",
  "extent": {
    "slat": 1.13,
    "nlat": 1.47,
    "llon": 103.59,
    "rlon": 104.07
  }
}