EGM2008 height - EPSG:3855

WKT

VERTCS["EGM2008_height", 
	VDATUM["EGM2008_Geoid"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["EGM2008_height", 
	VDATUM["EGM2008_Geoid"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "3855"]]

Data from @esri/proj-codes

{
  "wkid": 3855,
  "latestWkid": 3855,
  "macro": "PE_VCS_EGM2008_GEOID",
  "name": "EGM2008_height",
  "wkt": "VERTCS[\"EGM2008_height\",VDATUM[\"EGM2008_Geoid\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"EGM2008_height\",VDATUM[\"EGM2008_Geoid\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"3855\"]]",
  "description": "EGM2008 height",
  "authority": "EPSG",
  "version": "6.18.3(10.1.0)",
  "deprecated": "no",
  "areaname": "World",
  "extent": {
    "slat": -90,
    "nlat": 90,
    "llon": -180,
    "rlon": 180
  }
}