CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXYTYPE, long type);
DESCRIPTIONPass a long with this option to set type of the proxy. Available options for this are CURLPROXY_HTTP, CURLPROXY_HTTP_1_0 CURLPROXY_SOCKS4, CURLPROXY_SOCKS5, CURLPROXY_SOCKS4A and CURLPROXY_SOCKS5_HOSTNAME. The HTTP type is default.
If you set CURLOPT_PROXYTYPE(3) to CURLPROXY_HTTP_1_0, it will only affect how libcurl speaks to a proxy when CONNECT is used. The HTTP version used for "regular" HTTP requests is instead controlled with CURLOPT_HTTP_VERSION(3).
Often it is more convenient to specify the proxy type with the scheme part of the CURLOPT_PROXY(3) string.