Controlling volume on LG monitor

I now have an external LG monitor connected to my MacBook Pro. I'm using it as my only monitor with the lid on the MacBook closed. My keyboard therefore no longer controls the volume on the monitor anymore and I have to use the toggle switch on the bottom of the monitor itself. I read that I could set up a preset on BTT that could do this for me using my magic mouse (or reconnect it to the keyboard volume controls would be even better?). I tried to do it by creating a three fingered mouse swipe but it doesn't seem to work. Can someone walk me through how to set this up? Thanks.