Source

File: wp-includes/Requests/Proxy/HTTP.php

public function curl_before_send(&$handle) {
		curl_setopt($handle, CURLOPT_PROXYTYPE, CURLPROXY_HTTP);
		curl_setopt($handle, CURLOPT_PROXY, $this->proxy);

		if ($this->use_authentication) {
			curl_setopt($handle, CURLOPT_PROXYAUTH, CURLAUTH_ANY);
			curl_setopt($handle, CURLOPT_PROXYUSERPWD, $this->get_auth_string());
		}
	}