If you know what you're doing, it's pretty easy. If not, you can break things . . .
You can add a USB plug that will take battery voltage down to the 5V needed by your iPhone. Then use a short USB-to-Lightning cable to power the iPhone.
There are versions that plug into the existing 12V power socket on the bike (doesn't need any wring skills, but does leave some wires dangling), or you can hard wire to the battery for a neater, more permanent solution.
They make a waterproof USB plug
that works well. The waterproof isn't strictly necessary if you tuck the USB plug up under the battery cover and leave the cable plugged in all the time, but it's not a bad idea.
You'd still need to add a fuse, and if you hook directly to the battery then it would be live all the time, potentially draining your battery down if the bike sat for a long time.
A nice option is to find the existing GPS plug that sits right in front of the battery and use that to power the USB plug. You can get a BMW cable that plugs right in so you don't have to cut any of the bike's wires, then just add the USB plug to that. That would solve the fuse problem, and power the iPhone whenever the key is on, until 60 seconds after key off. The plug can be seen right near the steering pivot point in this pic:
You'd need BMW part # 83 30 0 413 585 REPAIR PLUG, 3-PIN - NO. 611656 which costs about $20, and a USB socket like the waterproof plug listed above. And a Lightning cable of whatever length fits between the battery and your iPhone location.
I'm down in Oxnard, so I'd be glad to help you out with all of this.