Oh parts of this remind me of having to write an HMAC signature for some API calls. I like to start in Postman, but the provider's supplied Postman collection was fundamentally broken. I tried and tried to write a pre-request script over a day or two, and ended up giving up. I want to get back to it, but it's frustrating because there's no feedback cycle. Every request fails with the same 401 Unauthorized error, so you are on your own for figuring out which piece of the script isn't doing quite the right thing.