curl/docs/cmdline-opts/_PROXYPREFIX.md

PROXY PROTOCOL PREFIXES

The proxy string may be specified with a protocol:// prefix to specify alternative proxy protocols. (Added in 7.21.7)

If no protocol is specified in the proxy string or if the string does not match a supported one, the proxy is treated as an HTTP proxy.

The supported proxy protocol prefixes are as follows:

http://

Makes it use it as an HTTP proxy. The default if no scheme prefix is used.

https://

Makes it treated as an HTTPS proxy.

socks4://

Makes it the equivalent of --socks4

socks4a://

Makes it the equivalent of --socks4a

socks5://

Makes it the equivalent of --socks5

socks5h://

Makes it the equivalent of --socks5-hostname