code: public function confirmationUrl()
      {
          // Begin the charge request
          $charge = $this->api->rest(
              'POST',
              "/admin/{$this->plan->typeAsString(true)}.json",
              ["{$this->plan->typeAsString()}" => $this->chargeParams()]
          )->body->{$this->plan->typeAsString()};
  
          return $charge->confirmation_url;
      }
docs: Gets the confirmation URL to redirect the customer to.
  This URL sends them to Shopify's billing page.
  
  Example usage:
  (new BillingPlan([shop], [plan]))->setDetails($plan)->confirmationUrl();
  
  @return string