Laravel Lumen: retrieving the request and the header values in the service provider class is not working

Question

I am working on a Laravel Lumen project. I am not trying to retrieve the header values from the request inside the AuthServiceProvider class as follow.

class AuthServiceProvider extends ServiceProvider
{
    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        //
    }

    /**
     * Boot the authentication services for the application.
     *
     * @return void
     */
    public function boot()
    {
        // Here you may define how you wish users to be authenticated for your Lumen
        // application. The callback which receives the incoming request instance
        // should return either a User instance or null. You're free to obtain
        // the User instance via an API token or any other method necessary.

        $this->app['auth']->viaRequest('api', function ($request) {
            if ($request->input('api_token')) {
                return User::where('api_token', $request->input('api_token'))->first();
            }
        });

        $request = $this->app->request;
        dd($request->header("api-key"));
    }
}

But it is not working. It is always returning null. I tried the following too.

request()->header('api-key');

That is not working either. But I can retrieve the values inside the middlewares or controllers. It is just not working inside the service provider class. How can I fix it?