Payment Gateway Callback URL

[ifeomai]

I'm trying to customize this for Nigeria using Paystack Gateway - https://paystack.com/ . Payment is completed on its site and Paystack requires a callback url to post the response status to. What would this be in this case? Are there any parameters to be included?

Reference - https://developers.paystack.co/docs

As an aside, is there any documentation / post on how to create or customize the gateways for unsupported countries?