I don't know where to let garmin know about the obstacles, but a couple of weeks ago I called up garmin to find out where they get their obstacle database from since my organization was looking to purchase either ForeFlight or Garmin Pilot. Our main concern is obstacle avoidance since we fly 300 ft and below for spraying.
I talked to someone that deals with the databases in Garmin and he couldn't tell me where they get the obstacle database. I also looked up Garmin's Database for obstacles and it was last updated in Feb 2015.