curl/docs/cmdline-opts/ntlm.md

c	SPDX-License-Identifier	Long	Help	Mutexed	Protocols	Requires	Category	Added	Multi	See-also	Example
Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.	curl	ntlm	HTTP NTLM authentication	basic negotiate digest anyauth	HTTP	TLS	auth http	7.10.6	mutex	proxy-ntlm	--ntlm -u user:password $URL

--ntlm

Use NTLM authentication. The NTLM authentication method was designed by Microsoft and is used by IIS web servers. It is a proprietary protocol, reverse-engineered by clever people and implemented in curl based on their efforts. This kind of behavior should not be endorsed, you should encourage everyone who uses NTLM to switch to a public and documented authentication method instead, such as Digest.

If you want to enable NTLM for your proxy authentication, then use --proxy-ntlm.