CURLOPT_PROXYUSERPWD man page
CURLOPT_PROXYUSERPWD — user name and password to use for proxy authentication
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXYUSERPWD, char *userpwd);
Pass a char * as parameter, which should be [user name]:[password] to use for the connection to the HTTP proxy. Both the name and the password will be URL decoded before use, so to include for example a colon in the user name you should encode it as %3A. (This is different to how CURLOPT_USERPWD(3) is used - beware.)
Use CURLOPT_PROXYAUTH(3) to specify the authentication method.
The application does not have to keep the string around after setting this option.
This is NULL by default.
Used with all protocols that can use a proxy
Returns CURLE_OK if proxies are supported, CURLE_UNKNOWN_OPTION if not, or CURLE_OUT_OF_MEMORY if there was insufficient heap space.
curl_easy_setopt(3), CURLOPT_PROXYAUTH(3), libcurl-tutorial(3).