I mean, that's true. I guess I'm so used to using fan controllers that run off Sata or Molex power that this would be a good argument.
OK, so backing up - do you have any adapters which could be used to power the pump from Sata, etc? Otherwise, you might have to do this the old fashioned way and actually try to boot your machine. This might come with a bit different set of configuration.
Disconnect SATA data cables to motherboard - helps prevent SSD/HDD issues with repeated start/stops. Could also remove m.2 SSD as well.
Could remove GPU, would be recommendation.
Paper towels everywhere on hardware (probably should be default for anyone anyway)
Fill reservoir as full as possible, leave uncapped.
Plug in ATX cable to motherboard.
Power on system - be ready to turn off power as soon as reservoir is almost empty.
Follow steps previously on fill/cycle until full. Once the loop can run constantly and move coolant, slowly fill reservoir until 90% or so filled, leave uncapped and run for few moments.
Power off PC.
Install GPU, drives/cables, etc.
Be mindful of paper towels, can leave most in place if you feel it is needed.
Power on, check system for system boot up. Should get to OS just fine.
Check coolant levels - let system run with reservoir vented for 30 mins or so.
Power down. Cap reservoir.
Power on, tilt and rock case to dislodge air to reservoir - vent reservoir as soon as tilting process complete.
After about 60 mins, cap and just keep close eye on system for a while. Might see occasional air bubble come to reservoir and displace coolant. Top off reservoir as needed.
If you are constantly topping off the reservoir, you likely have a leak.