Retrieiving Authorization Tokens from a third party

We are trying to retrieve an auth token from a third party system in order to pass Purchase Order's directly to their server.

var headerObj = {

        "Content-Type": "application/json",

        "EnterprisesID": "Account#"

      }

      var getAuthToken = https.get({

        url: 'https://lite.actionflow.net/Token?grant_type=password&userName=TheTestName&password=Testing123!',

        headers: headerObj

      });

The response we get is 4-00 Bad Request "unsupported_grant_type".

Of course the get request works in postman,

Any suggestions? Below is the explanation of the error from NS documentation