Hi,
When I wanted to have my Hella 4000s switch on with my high beams I connected to the fuse. There is power at the fuse only when the high beams are switched on.
I thought about tapping into the wire at the headlamp, but I was concerned about corrosion getting into the splice unless I soldered. Since I didn't feel like soldering, and I was going to put a switch on the dash anyway, I thought I might as well use the fuse.
Hope this helps.
Victor