Please check the API documentation. This might have to be part of the URL, as a header field, or as part of a request data structure which you send with your API call (usually in the form of a JSON or XML document). It might even by part of your account setup at the API provider.
You need to provide a callback URL to them which they use to call you back. When you handle their call, the fact that it is a callback is irrelevant. All yo have to do is receiving the data they provide, according to their documentation as to what information is sent to you in what form with that callback call. This is in no way different from providing a service that someone may use at their discretion, albeit usually in this case, you can dictate the interface. In the case of a callback they define the interface. BC may be able to implement that interface. Likely, though, it is not, and you will have to create a proxy, which passes on the information received by the callback.
Answers
so how do i provide this callback url?
i dont see any documentation about callbacl url from BC.
do you mind to share?
Thanks.