Would like to assign a session var and add to an https header

I'm completely new to F5/Big IP and have been assigned to work with our Network team to integrate an application SSO solution with the existing iRule. The network team only do basic re-directs and are intimidated at going beyond the working iRules. From my research this does not appear to be too difficult. What we need to do is assign a session variable to a local var and insert it into the header. I'm thinking it should look something like this:

when HTTP_REQUEST { set inbound_uri [string tolower [HTTP::uri]] set mycustom_var[ACCESS::session data get "session.logon.last.username"] HTTP::header insert $mycustom_var if { $inbound_uri starts_with "Blah" } {HTTP::redirect }

Any assistance will be much appreciated,