To be able to fully enjoy HTTP proxy protocol please go to privoxy website https://www.privoxy.org/, locate package for your OS (Windows, Mac, Linux) and download.
Privoxy will look for a file named config in the current directory. Except on Win32 where it will try config.txt so if this file is not present please create it in the directory where privoxy binary is located.
Add a following line to the Privoxy configuration file:
forward-socks5 / 18.104.22.168:50000 .
Replace port (50000 in the example) with socks5 port provided in the panel.
Then by default you can connect with curl as follow:
curl --proxy http://127.0.0.1:8118 https://www.localproxies.com/show_ip.php -v