For the last several years, iheartglaciers, I've been adding it as a note on my reservation (I make a loop request).
I call the reservation line and when I make a reservation ask them to add the request or if made online I call sometime in advance of arrival and ask them to add it.
Five days is the cutoff for room only cancellations whose deposits are refundable so that's when the Fort knows who is coming and for how long with some degree of certainty. The computer system pre-assigns a site number to your reservation based on what should be vacant on your arrival day and then the list gets tweaked with overnight changes and manual intervention (the system can't see your site request per se but the info gets passed to the Fort where a CM sees it and attempts to fulfill it).
Sometimes, though, it doesn't work out. For example if a prior guest checks in for a week on the site you want and the next day is your T minus 5 day arrival, the prior guest will still be on the site and you will get a different one. I've also personally encountered a guest who was supposed to leave and vacate the site that I wanted and that guest decided to extend their stay - since there was vacant capacity available, they didn't have to move. So of course it's just a request.
I can say, however, that when I've checked in (I like to do it in person), the CM has ALWAYS seen my request so the phone/reservation method does what it's supposed to do.
Bama Ed