Thanks for sharing that with me @chris14
It looks like you are using an Oauth app to authorize.
What I can see is that you are trying to authorize and get the token at the same time and it should be done in 2 steps.
First, you authorize the app (using the app credentials), once that’s done then you can generate the access token
Here is a helpful guide on how to make API calls with Oauth using Postman
Also, take a look at this sample app that might help you understand the workflow better