Post and get data using flutter webview - flutter

Question

I'm having a problem with Flutter. More precisely, since it is not a language that I have a command of, I did something by looking at the videos. Of course, old videos etc. I've been in trouble.
First of all, I will write what I want to do and what I failed to achieve. I am pulling my website into application with webview. There is no problem here. Then I added onesignal. This also works flawlessly. But I want to get the id called playerid that onesignal defines for each device and post it to my php page. I tried many plugins for this, but I could not succeed. Then I sent it as a cookie, it still didn't work. I realized that I can send data as get in the latest webview link, but the fact that I can't pull the variable I created here also infuriated me. I'm adding the codes below and asking for help from an expert friend. If there is, you can suggest an easy way to POST and GET. Thanks everyone in advance.
In the code below, I can't get the variable that I defined as userid and printed in the link section. In the simplest way, if I do this, it can work for now.
import 'dart:io';
import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'package:onesignal_flutter/onesignal_flutter.dart';
import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;
import 'package:dio/dio.dart';
void main() async {
runApp(PnlbaseApp());
}
class PnlbaseApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'PNLBASE APP',
home: PnlbaseAppPage(),
);
}
}
class PnlbaseAppPage extends StatefulWidget {
PnlbaseAppPage({Key? key}) : super(key: key);
#override
_PnlbaseAppPageState createState() => _PnlbaseAppPageState();
}
class _PnlbaseAppPageState extends State<PnlbaseAppPage> {
#override
void initState() {
super.initState();
OneSignal.shared.setLogLevel(OSLogLevel.verbose, OSLogLevel.none);
OneSignal.shared.setAppId("APP-ID");
OneSignal.shared.promptUserForPushNotificationPermission().then((accepted) {
print("Accepted permission: $accepted");
});
OneSignal.shared.getDeviceState().then((deviceState) {
print("OneSignal: device state: ${deviceState?.jsonRepresentation()}");
var userid;
userid = deviceState?.userId;
print(userid);
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: SafeArea(
top: true,
child: WebView(
initialUrl: "https://app.pnlbase.com/login?playerid",
javascriptMode: JavascriptMode.unrestricted,
),
),
);
}
}