Hi Ashutosh,
backend authentication (e.g. Basic Auth or NTLM) is normally transparent for the API gateway, means the corresponding requests/responses (e.g. 401) will be forwarded 1:1 between the real client and the backend.
But if you want that the API gateway behaves as client for this authentication process you can specify the credentials easily on the "Authentication"-tab of the "Route via HTTP"-Assertion:
See also the official documentation for this.
Maybe the wording "HTTP Credentials" is not optimal, but this is for Basic Authentication.
Hope I understand you correctly and could help you with this Information.
Ciao Stefan