Grant-Type issue on app with JWT Flow

I tried to generate the access token with  Postman, and I'm running into issues.

Here's what I wrote in Postman:

URL: https://platform.ringcentral.com/restapi/oauth/token

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

Body
grant_type: urn:ietf:params:oauth:grant-type:jwt-bearer
assertion: JWT token generated in the console for all apps under my Organization
client-id: WaOUXvk0oXtdlR79ZvmEZt
client-secret: c7Hp7saR............

{

    "error": "invalid_client",

    "error_description": "Client authentication is required",

    "errors": [

        {

            "errorCode": "OAU-123",

            "message": "Client authentication is required"

        }

    ]

}

Solved, I was missing the Authorization header ….

Authorization: Basic client_id:client_secret  #encoded in Base64 format

I hope this helps someone!