In Stripe, if you want to add a coupon code to a client's subscription and charge immediately [assuming there is a credit card already on file], here is the process to follow:

1. Log into Stripe
2. Go to the client's account
3. Under Customer, go to Coupons
4. Under Coupons, click Add Coupon and then add the coupon which you want to add
5. Under Customer, go to Subscriptions
6. Under Subscriptions, click Add Subscription
7. Then click Add Product
8. From the dropdown, click Winnow
9. Then select the plan:
    1. For default $99 use WINNOW_NEW_MONTHLY_DEFAULT_PLAN
    2. OR use plan according to the category
10. Review Information in the right-hand panel of the page
11. Click Start Subscription on the right top corner of the page