FortiWeb SAML Username

Hi, we've been trialling a FortiWeb VM to protect a few websites and we're using SAML to authenticate users and allow access to the sites hosted behind. The SAML is working ok, authenticating and allowing users as required to the sites, however checking the logs the username isn't being populated with the authenticated users, has anyone come across this issue before and know how to resolve it? We'd like to insert the usernames into a custom HTTP header to pass onto the web servers which seems possible under the site publish rule config, but without the username being pulled from the SAML authenticating in the first place it's not working, if anyone knows of a different way to achieve this it'd be a great help!

Thanks!