Report Options Specify Report File By Expression

Using TestStand 12; how do I set the report file path via a fileglobal variable within the sequence? I can set a static filepath by using the Report Options: 'Specify Report File By Expression' that seems to work, but what do I need to put in there for a variable filepath?

My guess is to use the ReportOptions Callback somehow but am unsure how; anyone managed to do this?