Applying coupons to Stripe Connect

調べた所PaymentIntentに対応していないらしく、割引を適用する場合は自分で対応してね、とのことです。

To implement a coupon or discount structure with Payment Intents, you will need to script logic on your backend to adjust the amount that is passed through the amount parameter to reflect the reduction when the Payment Intent is created.

参考記事

Support for coupons using Payment Intents API