.KTM's factory tool pushes the fluid in from the master cylinder reservoir. The reservoir adaptors vary from model to model, the pressurized pump/supply itself is German and quite pricey. I think our dealership spent about $2000 on the entire setup
FWIW you have to buy whatever tools KTM sends you
It doesn't matter which way you do it so long as you have the volume in your pump/syringe to completely fill the system w/o stopping.
I use a big syringe (100cc from a paramedic customer) to shove fluid through the bleeder bolt. I keep one marked "5.1" for brake fluid & another "oil" for the hydraulic clutch system where I use 5w fork oil/suspension fluid instead of the Magura or Motorex offerings.
I use a piece of safety wire looped around the plunger-end of the syringe body so I can keep it upright while fiddling with the process and not put any stress on the hoses- we've all had bleeder hoses pop-off in the middle of a job leading to air introduction nevermind the mess. Keep your syringe lines short so you don't waste or contaminate unused fluid. I take the plunger out and (with the lines connected & my bleeder wrench in place) fill the body with (whatever) fluid. Then I crack the bleeder bolt (not too far or you can suck air past the loose threads) to watch for the (usual) air bubble that comes out. Once the bubble has come to the top of the syringe, I place the plunger back in & begin to push down to push fluid into the system. Of course you'll want some dirty rags to catch the fluid that comes up out of the reservoir. Once it comes through clean close the bleeder bolt & you're done!
One advantage of the pressurized units forcing fluids through the system is air bubbles (which always stay in the highest loop of a line) can be pushed through- but this is dependant on the speed the fluid flows