here is chromium docs:https://chromium.googlesource.com/chromium/src/+/main/net/docs/proxy.md

your commandline "chrome --proxy-server="https://206.246.75.178:3129" not work may because you use a https protocol.

you can try to use this commandline: chrome --proxy-server="http://206.246.75.178:3129" which use a http protol proxy