Either seems to work. It's often best to have the tank full enough that the filter is running so there's some flow, then pour your little cup of water and remineralizing powder into the moving water.
Also I think you know this, but I like to use a
milk frother to quickly mix the cup of powders. They don't need to dissolve in the cup, just get hydrated.
Pouring that stirred mixture into your 90% filled aquarium with the filter running will quickly dissolve the mixture in a matter of an hour or so in most cases. If you don't, CaSO4*2H2O (Gypsum) will often clump and stick together underwater for up to 24 hours.
But none of this step is precise. Just mix the powders in the cup while you start filling your tank, add them to the water if/when the water has some movement, and do it all again in 7 days!