This API Reference document should provide the full URL for generating a RingOut:
https://developers.ringcentral.com/api-reference/RingOut/createRingOutCall
As for the full URL for retrieving a token... there are two resources I would direct you to. First is our RingOut QuickStart:
https://developers.ringcentral.com/guide/voice/quick-start/node
Using this Quick Start you could write a program that uses Password Auth to generate a RingOut. A trace will give you all the API Calls needed. 
The API Reference will break this down in more detail:
https://developers.ringcentral.com/api-reference/Get-Token
The full URL will be (for production):
https://platform.ringcentral.com/restapi/oauth/token
                
    
                                    
            I post this:
https://platform.devtest.ringcentral.com/restapi/oauth/token
and I get this:
{
  "errorCode": "AGW-404",
  "message": "Resource not found",
  "errors": []
}
What am I doing wrong?
(Note:  I'm using both the "Try It" button in the RingCentral instructional webpage, as well as the "Postman" app.  Both give the same result.