HTRS96 - ESRI:115891

WKT

VERTCS["HTRS96", 
	DATUM["D_Croatian_Terrestrial_Reference_System", 
		SPHEROID["GRS_1980", 6378137.0, 298.257222101]], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["HTRS96", 
	DATUM["D_Croatian_Terrestrial_Reference_System", 
		ELLIPSOID["GRS_1980", 6378137.0, 298.257222101]], 
	CS[vertical, 1], 
	AXIS["Ellipsoidal height (h)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["Esri", "115891"]]

Data from @esri/proj-codes

{
  "wkid": 115891,
  "latestWkid": 115891,
  "macro": "PE_VCS_HTRS96",
  "name": "HTRS96",
  "wkt": "VERTCS[\"HTRS96\",DATUM[\"D_Croatian_Terrestrial_Reference_System\",SPHEROID[\"GRS_1980\",6378137.0,298.257222101]],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"HTRS96\",DATUM[\"D_Croatian_Terrestrial_Reference_System\",ELLIPSOID[\"GRS_1980\",6378137.0,298.257222101]],CS[vertical,1],AXIS[\"Ellipsoidal height (h)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"Esri\",\"115891\"]]",
  "description": "HTRS96",
  "authority": "Esri",
  "version": "13.2.0",
  "deprecated": "no",
  "areaname": "Croatia",
  "extent": {
    "slat": 41.62,
    "nlat": 46.54,
    "llon": 13,
    "rlon": 19.43
  }
}