1. Create or edit a coupon and switch on Apply Discount to Recurring Subscription Rate.
2. Type must be Amount in Cart — Fixed Dollar Amount ($) or Percentage of Cart (Subtotal) (the wallet types are greyed out). Enter the Value.
3. Under Allow on, the page automatically ticks only Weekly Subscription and locks the other ordering systems off.
4. Save.

When a customer subscribes using the code, the subscription is stored with its original price and a discounted price (original minus the amount, or original × (1 − %)), and every renewal is billed at the discounted price. The coupon also still discounts the first checkout like a normal coupon. Only the first recurring coupon in a cart is applied to the subscription.

Saving fails with "Recurring subscription rate discount only works with Fixed Dollar or Percentage types." if you pick a wallet type, and this option cannot be combined with Exclude Meals from Coupon.