The request was formatted as json. You can send it as x-www-form-urlencoded if you format it that way. Since you already have it formatted as json, you can just use application/json.
You should be using the client id and client secret pair from the application that is registered for use with the management api.
You can find documentation here:
There is a button on there to show you the C# code you would run. A little further down is a table that explains where to get the values from.