You have it backward. You don't want to add the reservation to your MR#, you want to add your MR# to your reservation.
When you add the reservation to your MR account, doesn't your MR# get added to your reservation automatically?
Anyhow, while trying to add my MR# to my reservation, it says:
"We experienced technical difficulties while adding your Marriott Rewards number to this reservation. Please try again later or call 1-800-MARRIOTT."