title: How do you link customer to subscription on Stripe checkout?
body: <p>The problem I have is that everytime the customer checks out, they have a new customer id that is linked to the customer's email.</p>
  
  <pre><code>var stripe = Stripe('pk_test_xxx', {
      betas: ['checkout_beta_4']
  });
  
  var checkoutButton = document.getElementById('checkout-button');
  checkoutButton.addEventListener('click', function () {
      stripe.redirectToCheckout({
          items: [{
              plan: 'plan_xxx',
              quantity: 1
          }],
          customerEmail: 'test15@xxx.com',
          clientReferenceId: 'cus_xxx',
          successUrl: window.location.protocol + '//domain.test/en/accounts/billing-success',
          cancelUrl: window.location.protocol + '//domain.test/en/accounts/billing-cancel',
      }).then(function (result) {
          if (result.error) {
              var displayError = document.getElementById('error-message');
              displayError.textContent = result.error.message;
          }
      });
  });
  </code></pre>
  
  <p>I thought the <code>clientReferenceId</code> would persist the Stripe customer id. Seems like that is not the case. The subscription has a new customer_id.</p>