The code is valid for 120 days and needs to be entered via Mogstation - see screenshot in this very thread at post #10. But indeed you can't do that with trial accounts - you need to have the full game purchased to be able to add the code. Unless by "trial" you mean the first free 30 days of sub included with the game. Finally - the RAF code could be redeemed only by accounts which has not yet paid for / added any extra subscription days.