You should include a {CHECKOUT_SESSION_ID} in your return_url, for example: "https://localhost:44389/blah/StripeCheckoutReturn?session_id={CHECKOUT_SESSION_ID}". Upon completing a checkout session, Stripe will replace the {CHECKOUT_SESSION_ID} placeholder with the actual checkout sessions ID, so that your app can retrieve the Checkout Session ID from the query param. You can find example code here