Unable to use socks5 proxy built by ssh tunneling

[Chang Liu]

I am trying to use openconnect to connect a vpn server, through a
socks5 proxy that I build by ssh tunneling. Then when I try to
connect, it cannot connect to the https server. For example,

openconnect -v --proxy=localhost:1337 www.google.com
POST https://www.google.com/
Attempting to connect to server [::1]:1337
Connected to [::1]:1337
Requesting HTTP proxy connection to www.google.com:443
Error fetching HTTPS response
Failed to open HTTPS connection to www.google.com
Failed to obtain WebVPN cookie

I have tested the socks5 proxy in other applications. My guess is that
openconnect treat the proxy as a http proxy rather than a socks. But I
don't know how to set up the correct configuration.

The OS is Windows 10.