Laravel Passport - Grant Type Not Supported

It appears that you must send the parameters as form data and not in the headers like I was doing... Rookie Mistake!

I'm using Postman and I have put all parameters in Params. Postman shows the following response

{
    "error": "unsupported_grant_type",
    "message": "The authorization grant type is not supported by the authorization server.",
    "hint": "Check the `grant_type` parameter"
}

Now I put all parameters in Body and then press the Send button, It's working well.