If you’re reading this page, probably you ran across the same problem, which I had experienced. Unfortunately, there is no a direct way to solve it.
The community has been asking for this feature for 2 years so far, but in vain. But the discussion in that thread contains a workaround. What do you need is to download Fiddler app. It’s a web debugging proxy. Run the app, then in Rules menu tick Automatically Authenticate option. After this Postman will work with NTML authentication like a charm. One drawback, though, you have to keep Fiddler opened all the time while you’re working with Postman.
Also one more workaround can be found in the same thread. But it requires adjusting the system’s Internet options and adding the endpoint into Trusted sites, which is not an option sometimes.