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 is used - beware.)
Use CURLOPT_PROXYAUTH to specify the authentication method.
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