Yes for getting token you need to encode both client Id and client secret in base64 format in Authorization header in normal API call as follows:

Authorization: Basic <clientId:client secret in base64>

Here are several examples of how to use Authorization Flow in a variety of languages: https://github.com/grokify/ringcentral-demos-oauth

Also oauth url should be http POST

Are the < and > needed?

I am creating a http get and passing the Client_ID parameter

What is the exact string I need to encode for the parameter in my get?

Client_ID =base64encode("Authorization: Basic <clientId:client secret>")

Header should be named Authorization and the value will be :

Basic client id: client secret ( here client I'd +client secret value with colon in base64 )

should I follow:

https://developers.ringcentral.com/api-reference/Authorization

Do I need any params (i.e. client_id, response_type, redirect_uri, brand_id, state, prompt) or is the "Authorization" : "Basic base64encode(client-ID:client-secret)" header only needed?

Yes you need to pass the parameters as well . But check which type of Authorization your application follows.. is it Authorization code or password flow.. based on that the parameters and oauth url will change

I am trying to use the Python API

https://github.com/ringcentral/ringcentral-python

Any pointers here - I do see an internal function for this purpose Platform._request_token but haven't been able to get it to work

I am using Authorization code

So it turns out I had a typo in the Client ID and now I am past this error.

I do have one followup question. Where do I get the "Application code grant" code to put into the post?

Does the /restapi/oauth URI on the dev portal have the ability to provide this?

You can check few examples here with python.. hope this will help you

https://github.com/ringcentral/ringcentral-python

https://ringcentral-tutorials.github.io/call-ringcentral-apis-native-python-demo/

https://developers.ringcentral.com/library/sdks.html