CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXYAUTH, long bitmask);
DESCRIPTIONPass a long as parameter, which is set to a bitmask, to tell libcurl which HTTP authentication method(s) you want it to use for your proxy authentication. If more than one bit is set, libcurl will first query the site to see what authentication methods it supports and then pick the best one you allow it to use. For some methods, this will induce an extra network round-trip. Set the actual name and password with the CURLOPT_PROXYUSERPWD(3) option.
The bitmask can be constructed by or'ing together the bits fully listed and described in the CURLOPT_HTTPAUTH(3) man page.