System Keys. works but is NULL

Hi, I refer to System Keys as described here

I am using several of these in a script like this

var user_data;
var user = event.request.payload;

user_data ={‘resource’: [ { ‘DFUserID’ :,‘FirstName’: user.first_name, ‘LastName’: user.last_name, ‘Email’, ‘KnownAs’: user.display_name } ] };

var send =“hiyerDB/_table/Users?fields=FirstName,LastName”, user_data)

However returns the email but, which I assume to be the ID field if one goes to Users in the DF Admin panel, returns NULL.

In my script I had assumed the line …

var user = event.request.payload

…allows access to all system variables starting with user (such as, user.first_name and therefore, but I now think it only allows access to those parameters that were POSTed in the registration process, which does not include since at the time of POSTing it is not defined.

So how do we access Indeed how do we access any of the other System Keys such as session.token for use in a script?