FortiWeb 7.6.3 - SAML Header Forwarding

Hello everyone, 

I am using FortiWeb Site Publish functionnality with a SAML authentification (Google).
I need to get some of those SAML attributes to my backend server. 

Following the documentation https://docs.fortinet.com/document/fortiweb/7.6.3/administration-guide/272565/offloaded-authentication-and-optional-sso-configuration 

Here is what I did in my site publish rule : 


So, my problem is that my attribute is nowhere to be found in any of my headers. 

Maybe the $USERNAME var is empty ? Would fortiweb still create the header and give it a blank value ? 

Thanks in advance to anyone willing to give me a hand :)