This worked for me on Linux Mint 19.
First, install “xbindkeys” and “xautomation” with :
$ sudo install xbindkeys
$ sudo install xautomation
Then see which mouse button to remap by running "xev | grep button" and press the button you want to remap in the xev window. For me, it was button '8'.
The above comes from https://www.howtoforge.com/tutorial/map-mouse-on-linux/, but that article references /etc/X11/xorg.conf which I had trouble making.