All properly set up hynautic steering systems I ve seen have a reservoir in the ER with pressure gauge, sight gauge and Schroeder valve
The steering is a SeaStar(teleflex) flexible helm. The Pump is located behind the steering wheel. This is behind the dash. There is no specific reservoir, it is just the pump itself. There is a cap on top of it, most of the time there is a remote fill tube, if you have a remote fill tube the cap is about 3/4 inch black plastic cap on top of the dash above the steering wheel. See the attached gif.