Use httpProxy from the config.

[WalterliTell]

We need to use a httpProxy because of our firewall. Freeform seems to ignore the httpProxy configuration.

Could you consider using Craft::createGuzzleClient() instead of new Client().

For example:
$client = Craft::createGuzzleClient([ 'headers' => [ 'Referer' => Craft::$app->getSites()->getPrimarySite()->getBaseUrl(), ], ]);

I'm currently adding the proxy manually in your ReCaptcha.php but that's not optimal for maintenance.
Maybe I'm missing something, if so please correct me :)

Kind regards