Where can I find the callback url?

[123ysys]

Hello. I was trying to set the API with WHMCS. When I click the "Refresh" button of the MOFH server on the "Servers" page, the WHMCS returned this error tip:

Error
Not Found
The requested URL /json-api/batch.php was not found on this server.

So I dived into the API Documents and see if I missed something. Then I was confused with the Callback URL. How can I find it? Should I create a callback file and put them into my WHMCS folder first? Also, what is the absolute path of "/json-api/batch.php"? Should it be the "http://panel.myownfreehost.net/" or my WHMCS domain?

Thank you.