From the cited thread, the terminology is that you don't want your 'certificate attached" yet. I haven't done this for the free night certs but I have done this for 7-night reservations from a travel package. I had to (because of a pending major devaluation) make a reservation 11 months in advance, didn't know for sure which property I wanted to use it at, so was actually allowed to make reservations at three diferent properties without the certificate attached, then when my plans were settled I called and had the two extra res. canceled and then had the remaining reservation attached.

Caution: A fairly significant percentage of the Customer Service folks do not know how to do this, and, in this case you need to get them to check with a rep with more experience or alternatively hang up and call again.