Long: preproxy
Arg: [protocol://]host[:port]
Help: Use this proxy first
Added: 7.52.0
---
Use the specified proxy before connecting to the ordinary proxy. Hence pre
proxy. A pre proxy must be a SOCKS speaking proxy.

The pre proxy string should be specified with a protocol:// prefix to specify
alternative proxy protocols. Use socks4://, socks4a://, socks5:// or
socks5h:// to request the specific SOCKS version to be used. No protocol
specified will make curl default to SOCKS4.

If the port number is not specified in the proxy string, it is assumed to be
1080.

User and password that might be provided in the proxy string are URL decoded
by curl. This allows you to pass in special characters such as @ by using %40
or pass in a colon with %3a.

If this option is used several times, the last one will be used.