Selenium select iframe

I am using laracasts/Integrated. I would like to focus on stripe iframe. But I can't do it with this package. Has anybody solve this already? My current code

use unops\Models\Project; use unops\Tests\FactoryUser; use Laracasts\Integrated\Extensions\Selenium; use Laracasts\Integrated\Services\Laravel\Application as Laravel;

class BasketCheckoutTest extends Selenium { use Laravel, FactoryUser;

/**
 *
 */
public function testUserDonatesToProjectViaStripe()
{
    $user = $this->createUser();
    $project = factory(Project::class)->create();
    //login user
    $this->seePageIs('/checkout')
         ->submitFormFindBy($this->findByXpath("//button[@class='stripe-button-el']"))
         ->focusFrame("//iframe[@class='stripe_checkout_app']")
         ->submitFormFindBy($this->findByXpath("//button[@id='submitButton']"));
           
    
    //pay with stripe
    //see thankyou page
    //use already created project, user
}

protected function submitFormFindBy($button, $formData = [])
{
    foreach ($formData as $name => $value) {
        // Weird, but that's what you gotta do. :)
        $value = ['value' => [$value]];

        $element = $this->findByNameOrId($name);
        $tag = $element->name();

        if ($tag == 'input' && $element->attribute('type') == 'checkbox') {
            $element->click();
        } else {
            $element->postValue($value);
        }
    }

    $button->submit();

    $this->updateCurrentUrl();

    return $this;
}

protected function findByXpath($xpath)
{
    try {
        return $this->session->element('xpath', $xpath);
    } catch (NoSuchElement $e) {
        throw new InvalidArgumentException(
            "Couldn't find an element, with a class attribute of '{$xpath}'."
        );
    }
}

protected function focusFrame($xpath)
{
    try {
        return $this->session->frame('xpath', $xpath);
    } catch (NoSuchElement $e) {
        throw new InvalidArgumentException('No frame found.');
    }
}

}

Response error is: WebDriver\Exception\NoSuchElement: Element not found with xpath, //button[@id='submitButton'] Unable to locate element: {"method":"xpath","selector":"//button[@id='submitButton']"}