Deep link of your mobile app (a URI with a custom scheme, for example yourbrand://checkout/callback, not an http or https address), to which the payment provider returns the customer after the payment in the provider's app. Pass it when Cashier is opened in a WebView of your app and the customer pays with a method that needs it, for example Revolut Pay.
Note: for Revolut Pay in a WebView, your app must intercept the deep link (on Android, also the links that start with merchant.revolut.com), return the customer to the same WebView with Cashier (not a new one), and check the final status of the transaction on your backend: the deep link is not a confirmation of the payment.