FortiADC SAML Username and Script

Hi, first post on here and hoping someone might be able to help, we've been setting up the FortiADC in front of a few Web Severs with SAML authentication provided by the FortiADC and a 3rd party IDP. The SAML is working for the most part however we need to retrieve the SAML username from the authentication and insert it into the HTTP header to be provided to the end web servers. It looks like we might be able to do this with Content Rewriting, however it's not populating the value correct, has anyone been able to get this working or knows what the correct variable would be to use in the Header Value field? The other option we have may be the scripting but I've no idea where to start with that one, Lua scripting language is beyond me! 

Any help or a nudge in the right direction would be much appreciated! 

Cheers!

James