I just spent 2 hours also trying to figure out homelink but finally did:
I just setup my 2018 M550ix and 2019 X5
1. Make sure your opener is compatible homelink @
www.homelink.com
2. Hold the outer two buttons (of the 3) on your mirror until a fast green light is flashing. This resets the settings.
3. Pick a button to set up your garage door of the 3 on the mirror. I used the left button. Hold your remote 1-3" from the mirror and push the hand held garage opener remote button and the mirror button until the orange light on the mirror turns to a fast green flashing light.
*Mine did not work when the green was a solid light. It has to be flashing
4. I then had to get a ladder and go to my garage opener unit and hit the code reset button. Mine turned to a blue light on at your opener then turned a flashing red. WHEN the red is flashing hit your car mirror button 4 or more times until the garage door works.
*You have to do this fast as the red button doesn't flash very long.
It's easier to do with 2 people
Also the car power has to be on. You cannot do this with the car off.
5. NOTE: I have a double garage door (one door for two cars). I have two hand remotes (one for each car) and only one garage opener attached to the garage ceiling.
*I had to use the SAME hand remote for both cars. Very important.
It all works great now.