The fluid was drained when I did the cleaning, so there was nothing left inside. First I removed the upper fluid plug with a 17mm hex key on a 1/2" drive socket I got from AutoZone.
EA includes 3 quarts of Royal Purple transmission fluid, so I'm not going to second-guess them on this. I found a great fluid filler at AutoZone, with a flexible hose, on/off twist valve and funnel.
I was able to stick it through the passenger-side saddlebag battery rack and into the top filler hole.
The Haynes manual says the transmission takes 2.5 litres of fluid, so I filled it exactly to that level. Nothing dribbled out, so I stuck the tip of my finger into the hole and felt the fluid was right up to the level of the hole. I put the plug back in with a decent tightness. The online forums all say that the plug can get so tight that you have to heat the transmission case up to red-hot just to get the plug out, so I was careful not to overtighten it.