You can enable Basic Authentification in IIS Settings, then in postman, Authorization --> select Basic Auth type and set your account name and password.
http://ibtissamchabiba.blogspot.com/2017/03/solution-for-401-unauthorized-error.html
The above approach will not work until you are passing credentials or the authentication token in the request. We have another one, Request digest value is used only to prevent the cross-site scripting and not to authenticate the user.
To authenticate the current logged-in user then you have to send one more header to the request to pass the credentials as:
xhrFields: { withCredentials: true }
And also you have to enable the URL Rewrite option in the IIS to prevent the preflight option from getting dropped. Please refer this LINK to get it done.
If the above approaches, doesn't help you, then kindly refer the below links,
How to perform POST operations in SharePoint 2013 using REST API from external Application
Authenticating to SharePoint from fiddler/postman fails with 401