CURLOPT_LOGIN_OPTIONS (3) - Linux Man Pages
CURLOPT_LOGIN_OPTIONS: set login options
NAMECURLOPT_LOGIN_OPTIONS - set login options
DESCRIPTIONPass a char * as parameter, which should be pointing to the zero terminated options string to use for the transfer.
For more information about the login options please see RFC2384, RFC5092 and IETF draft draft-earhart-url-smtp-00.txt
CURLOPT_LOGIN_OPTIONS(3) can be used to set protocol specific login options, such as the preferred authentication mechanism via "AUTH=NTLM" or "AUTH=*", and should be used in conjunction with the CURLOPT_USERNAME(3) option.