Recent trip to a JW Marriott proved that the virtual concierge works BUT someone at the hotel has to read it and respond. I specifically requested a bottle of wine to be in the room upon arrival. Since it wasn't, I went to the desk. Their response .... "you're right sir, it's right here on the computer." My response ...... "knucklehead."
Might be better to call ahead.
When you book up directly through the Marriott website they will send out an email prior to your stay so that you can make your requests (normally around 5 days before hand)..
I always reply to ensure that we receive a quiet room and if need be to ensure that we will have a rollaway bed in the room if there is only one bed in the room.
It has been good for us, as the hotel has either replied by email to the requests or if they haven't replied we have received our requests on checking in.
You can also make other requests such as: reservations for the hotel restaurant and other payable items such as flowers, champagne etc.