Stripe API - add new product subscriptions

Stripe-AddNewSubscription.cs

//Register user for Product Plan in Stripe
                StripeConfiguration.SetApiKey(StripeAPIKey);

                var items = new List<StripeSubscriptionItemOption> {
                    //Subscribe to Stripe plan with corresponding ID - ENSURE YOUR STRIPE PRICING PLAN ID IS IDENTICAL TO THE APIM PRODUCT ID
                    new StripeSubscriptionItemOption {PlanId = Request.QueryString["productId"]}
                };
                var options = new StripeSubscriptionCreateOptions
                {
                    Items = items,
                };
                var service = new StripeSubscriptionService();

                //Get user's Stripe ID by retrieving object from user store
                var user = await UserManager.FindByIdAsync(Request.QueryString["userId"]);
                //Create a plan subscription for that user
                StripeSubscription subscription = service.Create(user.StripeId, options);

                if (subscription.Status == "active")
                {
                    //Register user for product in APIM
                    using (var client = new HttpClient())
                    ...