CURLOPT_PROXYHEADER man page
CURLOPT_PROXYHEADER — custom HTTP headers to pass to proxy
#include <curl/curl.h> CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXYHEADER, struct curl_slist *headers);
Pass a pointer to a linked list of HTTP headers to pass in your HTTP request sent to a proxy. The rules for this list is identical to the CURLOPT_HTTPHEADER(3) option's.
The headers set with this option is only ever used in requests sent to a proxy - when there's also a request sent to a host.
The first line in a request (containing the method, usually a GET or POST) is NOT a header and cannot be replaced using this option. Only the lines following the request-line are headers. Adding this method line in this list of headers will only cause your request to send an invalid header.
Pass a NULL to this to reset back to no custom headers.
Added in 7.37.0
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
curl_easy_setopt(3), CURLOPT_HEADEROPT(3), CURLOPT_HTTPHEADER(3).