Thanks for being part of the Visa Developer Community! Once you register a callback URL for a Project, Visa Developer issues an HTTP POST request to the URL specified every time that event occurs. The request's POST parameters will contain XML/JSON data relevant to the event that triggered the request.
On the API documentation page, on the right side of the browser click on 'API Reference'
On the API Reference page navigate to the right side of the browser and you will find the Code Explorer. Please use the Code Explorer to see the Request and Sample Response.
In addition to that, please take a look at our API Explorer. API Explorer lets you choose the Product API, Endpoint and Method to see the Request and Response. Here is the link to API Explorer - https://developer.visa.com/apiexplorer
Please let me know if you have other questions or need further assistance.