Did you try entering this at the very beginning screen? In this case, it will usually indicate on the outbound screen that it will apply the discount after you pick a return. If, after this, it says that it can't be applied, it will have a link to the fare rules. I think that this has some blackout dates, so you may want to check your dates against the full fare rules description.
Cheers.
I tried it before first then I tried to apply it after, I hit purchase and got a system error message. I dont see any blackout dates for this periods code thru Oct 26.