It's pretty easy, I did it with my bar. You're on the right track, all you need is a relay.
The Highbeam wire is one of the wires going into the headlight housing (obviously) and you can easily find it with a multimeter (if memory is correct, I think it's blue, but don't take my word for it, I'd have to check when I get home). I tapped it outside of the plug itself, as that's easier than messing with pins in a plug. The wire was doesn't have much give, so it's a bit of hand-scratcher to get the tap on. The wire tap will serve as your trigger, as - as long as it's hot, the relay will stay on, and thus the lights.
I used a posi-tap to tap the wire, but you can use any sort of tap. I find posi-taps so much easier and less "invasive" than other taps.
For those that don't know it already, a standard car relay is wired as follows:
Things you'll need:
1. Some extra wire (ideally a black one for the ground and some other not-red color for the trigger, I tend to use yellow for triggers so I can differentiate them) ($1)
Edit: Make sure the gauge is thick enough for the amps you need for the bar (probably no more than 10 amps)
2. A standard car relay ($5)
3. Crimping tool + connectors (a cheap tool, $5, connectors, $3)
So, to translate the wiring you need into that relay diagram:
Wiring:
1. The High beam tapped wire goes into pin 86.
2. A wire to either an existing tapped ground cable, or the negative terminal on the battery goes into pin 85.
3. Pin 87 connects to the positive (+) wire on your LED bar (the negative on the LED bar goes either to battery negative (ground) or some other ground). You can put a fuse in between them if you want to play things safe.
4. Pin 30 goes to the positive (+) pole on your battery (or some other battery source)
That will turn it on when the high beams go on.
I have mine additionally wired through some switches that let me control how and when it goes on, but that's probably overkill for what you're asking.
Edit: Found this picture later which is much better
