Find out Cookie in Flutter - authentication

Question

I use Flutter to make a HTTP-Post to a website to login. If I only send the data to login, the response of the HTTP-Post shows, that I`m not logged in. I tried to find out the cookie the Website sends. I did it with the software Postman. If I add the same cookie, I got at Postman, the HTTP-Post with Flutter works and the response shows, that Im logged in. After a while, the value of the cookie switched and the HTTP-Post via Flutter doent work.
How can I get the information about the actual value of the cookie from the website?
The code looks like this:
Future initiate() async {
ByteData bytes = await rootBundle.load('assets/Certificates/test.crt');
SecurityContext clientContext = new SecurityContext()
..setTrustedCertificatesBytes(bytes.buffer.asUint8List());
var client = new HttpClient(context: clientContext);
IOClient ioClient = new IOClient(client);
var form = <String, String>{
'user':'testuser',
'pass':'testpass',
'logintype':'login',
'pid':'128',
'submit':'Anmelden',
'tx_felogin_pi1[noredirect]':'0'
};
print(form);
var ris = await ioClient.get("https://www.test.de/login");
print(ris.headers);
http.Response res = await ioClient.post("https://www.test.de/login",body:form, headers: {
"Accept": "application/json",
"Content-Type": "application/x-www-form-urlencoded", "Cookie": "fe_typo_user=12345"}, encoding: Encoding.getByName("utf-8"));
ioClient.close();
client.close();
print(res.body.length);
return res.body.contains("logout");
}

Answer 1

This is how I get cookie:
static Future<String> login(String username, String password) async {
var url = _session._getUrl('/auth/login');
var map = new Map<String, dynamic>();
map['username'] = username;
map['password'] = password;
final response = await http.post(url, body: map);
var jsonResponse = convert.jsonDecode(response.body);
var cookies = response.headers['set-cookie'];
if (cookies != null) {
var csrf = _session._getCSRF(cookies);
LocalStorage.instance.setString('CSRF', csrf);
print(csrf);
}
return jsonResponse['msg'];
}