api call returns undefined offset:0

I am trying to make an external API call to my website. I create a createBill() method that looks like below:

public function createBill(){

        $option = array(
            'userSecretKey'=>config('toyyibpay.key'),
            'categoryCode'=>config('toyyibpay.category'),
            'billName'=>'Fuiyoh Hub',
            'billDescription'=>'Payment Credit',
            'billPriceSetting'=>0,
            'billPayorInfo'=>1,
            'billAmount'=>0,
            'billReturnUrl'=>route('toyyibpay-status'),
            'billCallbackUrl'=>route('toyyibpay-callback'),
            'billExternalReferenceNo' => 'Bill-001',
            'billTo'=>'example',
            'billEmail'=>'[email protected]',
            'billPhone'=>'01942411',
            'billSplitPayment'=>0,
            'billSplitPaymentArgs'=>'',
            'billPaymentChannel'=>'0',
            'billContentEmail'=>'Thank you for purchasing our product!',
            'billChargeToCustomer'=>2
          );  

    
          $url = 'https://dev.toyyibpay.com/index.php/api/createBill';
          
          $response = Http::asForm()->post($url,$option);
          //this line below throwing the error
          $billCode = $response[0]['BillCode'];
          dd($billCode);
         
          

          return redirect('https://dev.toyyibpay.com/'.$billCode);
    }

Route:

Route::get('/toyyibpay','ToyyibpayController@createBill')->name('toyyibpay-create');

When I tried to call the API using guzzle, I am shown an error:

Undefined offset: 0 
Upon further debugging, this line of code is the one that gives the error:

$billCode = $response[0]['BillCode'];

Based on the API docs, I am expected to get a sample result like:

[{"BillCode":"gcbhict9"}]

How do I retrieve the BillCode response?