It's not recommended to request a single, specific room as if this room is not available the room assigners won't know why you wanted that room. It is best to describe what you are looking for. For example corner room, 2nd floor close to food court.
I prefer to call and add our room requests to our reservation. There are room request options during Online Check In but they are predetermined and very limited, there is no free form box to add requests. I usually book our trips via phone and add our room requests when I book. But, calling and adding your room requests can be done any time from the day you book up until 4-5 days prior to your arrival.