The inline testing interface will let you authorize different ways based on the Open API spec. And, there is more than one “Authorize” button in the pop-up. If you’re using the testing tool you MUST authorize using the clientCredentials flow. authCode flow button was clicked instead.
If the inline testing in the documentation isn’t working, we recommend pulling the OpenAPI spec document into either Postman or Insomnia and using one of those tools for testing.