Hi, Steve,
I have a Liftmaster 3800 opener that uses the 398LM control panel. I found the instructions for modification on the Autocloser website. It's pretty easy to do if you have the soldering skills. Since I lacked the skill, I practiced on a junk board.
HTH,
Kevin
Thanks Kevin. Awesome, I have the exact same opener. Your image was enough for me to figure out the solder points.
Steve