Short URL

I'm going through the short URL design. I observed a behavior where you post a short URL in browser and it will automatically expands to longURL

I thought, there will be an API in shortURL application that has been exposed and when it gets invoked it returns a longURL which browser loads.

Surprisingly, when I tried hitting short URL with GET method in postman, I got HTML page in response. Is there a way that you can find longURL through an API?