Answer
IFS Cloud doesn't support Basic Authentication by default, and enabling it isn't recommended for security reasons even where technically possible — that's why the Postman call specifically fails while the browser succeeds. The browser call works only because you're already logged into the IFS Cloud UI in that browser, which has already handled OAuth token acquisition behind the scenes for that session (this is also why it wouldn't work from a fresh incognito tab with no active login). For a tool like Postman, you need to fetch an OAuth access token yourself first (for example, via the Direct Access Grant flow IFS documents specifically for integration tooling like Postman) and use that token for the API call, rather than sending Basic Auth credentials directly.