See these tutorials. Choose a language you are familiar with

https://developers.ringcentral.com/tutorials.html

Choose the Password Flow topic and select one of the tutorial from the "Access RingCentral platform services using native APIs" group.

"Can anyone explain are these request headers or request parameters? I'm myself confused and documentation pages seem to be scattered? Is the value in Authorization key is base64 encoded clientid and client secret or the username password combination? "

Header:

Authorization header is Base64 format of clientId and client secret separated with ' : ' as follow:

Authorization Basic <clientId:clientSecret in Base64>

Content-Type: application/x-www-form-urlencoded

url: https://platform.devtest.ringcentral.com/restapi/oauth/token

Body parameter :

username=<user name>&password=<password>&extension=<extension>&grant_type=password