Nifi API Authentication - Access Token

Question

I'm attempting to collect details about all processors on two installed instances of Nifi, versions 1.19.0 and 1.20.0. I want to use Nifi to do this so I can perform some minor ETLs and save the resultant dataset in a database. Both instances are configured as standalone authentication with SSL.

However, when I configure the InvokeHTTP processor how I think it should be to aquire a token, I'm getting a 400 status code and a response.body of "The username and password must be specified." on both instances.

Here's the super basic flow: Nifi Flow

Here's what the error looks like: Attributes of failed response

Here's the current config of the InvokeHTTP processor on the 1.20.0 instance which we can focus on from here out since they are both responding similarly.

Postman - InvokeHTTP Config

When I run the request with Postman, I get the expected token in the response body. And I'm able to make subsequent requests (the above config was a response from https://nifi06:8443/nifi-api/processors/39870615-0186-1000-e7d7-c59fa621ca7d in Postman).

I've tried the following:

1. Adding dynamic attributes for username/password as in the above configuration.
2. I've added them as JSON to the preceding Generate FlowFile processor's custom text.
3. And I've also tried using the Request Username/Request Password properties of the InvokeHTTP processor.

All return with the same response.body of "The username and password must be specified."

This seems it should be really simple and I'm sure I've been staring at it too long to see what I'm missing. Can anyone tell me where I need to specify the username/password?

Thanks.