How Submit Sales Invoice detail to 3rd party using API?

Hi all,

My scenario is pretty similar to this :

The 3rd party apps provide a SDK that comes with their API:

The overall picture is, we are requires to submit invoice detail (from ERPNext Sales Invoice) to that 3rd party apps using an API. They will respond back to us by confirming the submission. The authentication is using OAuth. But I am not sure which steps should I follow. Does this requires changes in internal source code as well ? Because I don’t have the access to do it.

What I did was OAuth Provider Settings, and OAuth Client. But I am lost. Do I need to touch on Webhook too? This is quite confusing for me. Can someone point out what should I do in brief ?

I refer to this: