The only thing I found on that was from a no-no site. Apparently it is a hidden action sent from the button on the quest box to the server, so it's not really useable in macros. However, it is useable in the UIBuilder. People who make the UI Mods out there use these interface actions for things like enter to loot, enter to accept and such. I've had to resort to Mayve's Accept/decline mod, which works well enough, but I wish there was a way to macro it myself.