HTTP request payload

I am sending an HTTP request to cache class:

s rObj=##class(%Net.HttpRequest).%New()
s rObj.Server="www.xyz.com"
s rObj.Https=1
s rObj.SSLConfiguration="xyz"
s rObj.ContentType="application/json"
d rObj.EntityBody.Write("payload in json format")

s sc=rObj.Post("/csp/ns/xyz.cls")

The request gets to the OnPreHTTP class method 

Class xyz Extends %CSP.Page [ ProcedureBlock ]
{

ClassMethod OnPreHTTP() As %Boolean
{
    q 1

}

}

The question is how to retrieve the request payload in the OnPreHTTP method.