irule - Proxy Protocol - always good to learn something new!

Hi All,

So i've implemented this irule
https://community.f5.com/t5/crowdsrc/proxy-protocol-receiver/ta-p/280518

It's working brillantly and i'm learning a lot about it already.

But i need some advice, what i need it to do is add a HTTP Header of X-Forworded-For of the $v2sourceAddress so i can pass that through to ASM.
But i'm getting a error of "command is not valid in current event context", now my gut feeling is that this is because the irule is working at the client conenction / tcp level and not the HTTP profile level to understand that the flow is HTTP and that a header can be added.

Can anyone suggest how i can add to this iRule to add this functionality please? 
I'm stuggling to find any relevant information on if my gut feeling is correct let alone how to solve it.

Thanks all - Fletch