Can the 3rd night free cert be used in conjunction with paid stays booked thru an online travel agency? For example, can I book 2 nights via Expedia and then call the Concierge line to add the 3rd night free to my reservation?

Or do you have to call the Concierge line to book the paid stays and redeem the 3rd night free cert at the same time?

Asking because the Fairmont Orchid is currently much more expensive on the Fairmont.com website than on OTA websites (nearly $200/night difference). Any advice would be greatly appreciated!