I do it by phone and it works.
"Do you have an SX code?" they ask
I mention SX797398
then right afterward or at the end of the reservation they ask "agency/account number?"
I mention 597783
never had a question about it....
Enjoy!
I however never give my platinum card at the beginning nor at the end of the phone call because the rate i get with it is always higher.
However when I get to the location spot i give it so that the paper filling goes faster as well as the upgrade!!!