Creating a custom network in command line, just four lines above: https://github.com/linuxserver/docker-letsencrypt/blob/master/root/defaults/proxy-confs/_readme#L24
The preserve setting is under docker settings, advanced view (see screenshot). The rest is in container settings. It says "Then in each container setting, do one of the following:" so you need to look for that option in container settings, not docker settings.
I recommend reading the entire document first, before attempting changes so you understand all the steps first.