You are correct. The 'referrer' is a header so you need to use Header.Request.Get. Take a look at the default access.log and you can see how user-agent is retrieved and follow that convention. Don't forget to update your header line if pushing to Web Reporter.
You want the value from Header.Request.Get ("Referer")
You can add it to the log line via:
String.ReplaceAll (String.ReplaceIfEquals (Header.Request.Get ("Referer"), "", "-"), "=", "\=")
NOTE: the reason for this specific formatting is that the logs are going into a SIEM which has a standard that states that = signs should be escaped with a \ character.
This worked for me
+ Header.Request.Get ("Referer")