Thanks for sharing more information.
From what I’ve seen, it doesnt look like we have enough to go off of.
Before proceeding further, it would be worthwhile to change your pre-request script to use a POST instead of a GET Method, as noted by @allen.helton. This is even mentioned in the documentation you referred to:
When the user submits the form, the browser executes a POST request with the information.
If that doesnt work, the best course of action is to investigate the HTTP request in transit. Using Chrome Developer Tools (or whatever browser dev tools), you can do this, and simply record the network traffic before authentication, and then stop it. You should see how the HTTP request is formatted, and that should provide enough insight as to what you need to create within Postman.
In addition to that, I noticed in your original post that you have a request body in JSON:
Would you happen to know how that JSON actually looks? Chances are thats the body you need in your Pre-Request script too, in order to perform your authorization.
With Basic Auth (generally speaking), you dont need to get a token as the token itself if the combination of
base64(username + ':' + password) thats used in every request that need authorization. With the access token, your web service should be providing that for you upon authentication at the auth endpoint.
If you can provide those extra details mentioned about, we can get closer to authenticating to your web service