Custom Reports

I need a custom report (APM/Event Logs/Access System Logs) I want to add a session variable to the report but I'm not sure I am doing this correctly. Is the "session variable name" field intended for adding session variables that are not among those in the list?

 

In a report I've created, I've selected fields that I want and then the last field, session variable name, I have entered the name of a session variable that I want, but I get no data. I've tried using session.server.landinguri, I can see data in Manage Sessions for this variable.

 

Am I doing this right?