I don’t use Postman, but my advice would be to skip whatever auth process they use and just properly go through the OAuth steps as documented https://dev.twitch.tv/docs/authentication and then you can properly set the headers yourself to make the requests. This still can be done in Postman if you wish, just without their auth stuff getting in the way.
The documentation has step by step instructions for each of the OAuth flows. If you don’t require any scopes, and this is just for testing then the Client Credentials flow may be ideal as it’s very simple and gives you an App token which will work for requests that don’t require any user permissions.