DVC members should contact member services for requests. Chat agents are limited in what they can do, so that's often not the best path. Either call Member Services directly or use the "Contact Us" email form on the member website.
I can't speak for every resort but there is usually a requests component to the online check-in process. It doesn't jump right out at you and is easy to overlook. But it is also VERY limited in options that are offered. I recently did an online check-in for Poly and I think the only requests that could be selected were for Moorea building or 1st floor.
Also if you enter ANYTHING on the request section during online check-in, it supposedly overwrites any request details previously entered.
Bottom line: call member services or email.