curl -u '{clientId}:{clientSecret}' -d 'grant_type=password&username={username}&extension={extension}&password={password}' -X POST https://platform.devtest.ringcentral.com/restapi/oauth/token

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

Headers:

"Accept":"application/json“

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

"Authorization",:"Basic <ClientID:ClientSecret in base 64> 

Here you can get the clientID and clientSecret from your developer account portal under dashboard tab which needs to be converted into base 64 format.

In body, we need to pass the parameters in following way :

username=<account phone number>&password=<account password>&extension=<your extension>&grant_type=password

username will be the phone number associated with the account. Again, you will get that from dashboard.