dart – http api call via proxy for flutter

Currently my server has already setup the proxy, when i wanted to call external api it will be block, so right now i want to call the api www.google.com/recaptcha/api/siteverify to implement captcha to my project

final response = await http.post(
      Config.verificationURL,
      headers: {
        'Content-Type': 'application/x-www-form-urlencoded',
        'Access-Control-Allow-Origin': '*'
      },
      body: 'secret=${Config.secretKey}&response=$_token',
    );

any idea on how to convert this api call via proxy for flutter?