"Invalid resource owner credentials" for Password Flow via Postman

I have an app with Platform type as "Password Flow: and "Refresh Token" . I am using postman to access token and I get below error.

{
  "error": "invalid_grant",
  "error_description": "Invalid resource owner credentials",
  "errors": [
    {"errorCode": "OAU-140","message": "Invalid resource owner credentials"}
  ]
}

Below is what I have tried:

1. https://platform.devtest.ringcentral.com/restapi/oauth/token? along with parameters - username, password, extension and grant_type=password.
2. Auth - Auth Basic used username = Client_id and password =Client Secret.

I do see authentication is added in header.

I have tested auth using the link below, using the same parameters and client id/secret.

http://ringcentral.github.io/api-explorer/#!/Authentication/oauth_token_post

Could you please help me in fixing the error.

Kindly let me know if I am using incorrect method.

Thank you.