Thanks to the overloads for AddBody that is not a big challenge:
{ as Json string }
Request.AddBody(TJSON.ObjectToJsonString(aMsg, [joIgnoreEmptyStrings, joIgnoreEmptyArrays, joDateIsUTC, joDateFormatISO8601]));
{ or as TJsonObject }
Request.AddBody(TJSON.ObjectToJsonObject(aMsg, [joIgnoreEmptyStrings, joIgnoreEmptyArrays, joDateIsUTC, joDateFormatISO8601]), ooREST);