Flutter call back url

I’m using a dependency to impliment a payment system in my flutter app using Paystack Payment system. This is the dependency pay_with_paystak

In the dependency it’s expeting me to give a callbackUrl. What is this callbackUrl and how do I create one for both Android and iOS?

PayWithPayStack().now(
    secretKey:
    "sk_live_XXXXXXXXXXXXXXXXXXXXXXXXXXXX",
    customerEmail: "popekabu@gmail.com",
    reference:
    DateTime.now().microsecondsSinceEpoch.toString(),
    callbackUrl: "setup in your paystack dashboard"//How do I do this?
    currency: "GHS",
    paymentChannel:["mobile_money", "card"],
    amount: "20000",
    transactionCompleted: () {
        print("Transaction Successful");
    },
    transactionNotCompleted: () {
        print("Transaction Not Successful!");
    });