Listen to "paid" message of prompt.cash iframe and navigate parent page to "return_url"

prompt.js

(function () {
    const gatewayOrigin = "https://prompt.cash"; // ignore messages from other frames
    const onMessage = (event) => {
        if (event.origin !== gatewayOrigin) {
            return;
        }

        let data = event.data;
        if (data.func === "onPromptPaid") {
            // the iframe says the payment is confirmed
            // note that a malicious script in a browser with CORS security disabled might be able to trigger this
            // You should not rely on this message and instead the Callback to your server or use Prompt.Cash REST API to confirm:
            // https://prompt.cash/pub/docs/
            const payment = data.data;
            window.location.assign(payment.return_url);
        }
        else
            console.debug("Received unknwon Prompt.Cash message type: %s", data.func);
    }

    if (window.addEventListener) {
        window.addEventListener("message", onMessage, false);
    }
    else if (window.attachEvent) {
        window.attachEvent("onmessage", onMessage, false);
    }
})();

This code belongs into the parent page (the one including the frame).

The above example reloads the parent page to the return_url you specified in /create-payment. You can replace window.location.assign() with your code as needed (call your own backend to update the payment status, ...).

All properties of the payment object are listed here: https://prompt.cash/pub/docs/#create-a-new-payment