How to Integrate OneSignal in Laravel to Send Push Notifications to a Flutter App?

i have some questions..

How do I set up OneSignal in Laravel? What packages or libraries should I use?

is it possible to do via onesignal rest api?

currently i have tried this but failed?

OneSignalService

<?php

namespace App\Services;

use GuzzleHttp\Client;

class OneSignalService
{
    protected $appId;
    protected $apiKey;
    protected $client;

    public function __construct()
    {
        $this->appId = config('services.onesignal.app_id');
        $this->apiKey = config('services.onesignal.api_key');

        $this->client = new Client();
    }

    public function sendPush($title, $message, array $playerIds = [])
    {
        $url = 'https://api.onesignal.com/notifications?c=push';

        $headers = [
            'Authorization' => 'Key ' . $this->apiKey,
            'Content-Type' => 'application/json',
            'Accept' => 'application/json',
        ];

        $payload = [
            'app_id' => $this->appId,
            'headings' => ['en' => $title],
            'contents' => ['en' => $message],
            "target_channel" => "push"
        ];

        if (!empty($playerIds)) {
            $payload['include_subscription_ids'] = $playerIds;
        } else {
            $payload['included_segments'] = ['All'];
        }

        try {
            $response = $this->client->post($url, [
                'headers' => $headers,
                'json' => $payload,
            ]);

            return json_decode($response->getBody(), true);
        } catch (\Exception $e) {
            return ['error' => $e->getMessage()];
        }
    }
}

Route::get('/test', function () {
    $users = User::whereNotNull('onesignal_player_id')->get();

    $playerIds = $users->pluck('onesignal_player_id')->toArray();

    if (!empty($playerIds)) {
        $response = app(OneSignalService::class)->sendPush(
            "Hi there",
            "This is a test notification",
            $playerIds
        );
        dd($response);
    }
});

and i cant figure what type of token/id should i get/save from flutter? to user model?

Any guidance, code examples, or links to relevant documentation would be greatly appreciated!