The get method has the [Authenticate] attribute on it.
The integration test with AppSelfHostBase and JsonServiceClient that passes username and password works with no problem.
I checked on IIS and Basic Authentication is enabled and it is configured also in the host configuration.
If I try to pass the username and password in the url or I try to put the basic authentication on Postman (headers) I get the unhauthorized 401 error.
It should just work the server doesn’t care about which client sends the request just the HTTP Headers sent with the request.
Try deleting the ss-id/ss-pid Cookies, in Fiddler you can drag the Request into the Composer and replay the request, try it without sending existing cookies.
Also note the special support for Authentication in Postman basically it lets you export your authenticated session into a url that you can paste into Postman to copy the Session cookies.