Put a relay that gets power from the high beams. That basically turns on a switch when the high beams are on. The switch is the other side of the relay. So the two wires not being used by connecting them to high beam power, connect one to the battery. Fused of course. And the other to the lights, then they'll come on and off with the high beams.
Now if you want to get really fancy you can add a switch in the car to turn the hellas off. Off as in, they don't come on with high beams, or on, as in they do come in with high beams. I haven't done that with my setup yet just because I'm lazy and hate wiring things into the cab of my car, but it isn't that hard.