Error when requesting access token from Vend API for test application

I'm trying to obtain an access token for a test application using the Vend API by sending a bunch of parameters in the body of POST request of type "application/x-www-form-urlencoded". I've already obtained an authorization code however I keep on receiving an error response. I'm using Postman to send the request and it appears that the Vend API makes use of OAuth2.

The response - 400 Bad Request:

{
    "error": "invalid_request",
    "error_description": "The request is missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or is otherwise malformed. Check the \"grant_type\" parameter."
}

The error seems to suggest the problem is the "grant_type" parameter, I've tried assigning it other values such as the authorization code and redirect URI to no avail. I'm now suspecting that the problem is elsewhere.

POST request URL:

https://samplepsyirf01.vendhq.com/api/1.0/token

POST request parameters:

code: KWDZNSo67gnf82PHa805k_kLlcq7VYZHQeAhoK7J
client_id: AdP6KYWqPjM7WbkDnNafoJ5piI6GECRa
client_secret: {CLIENT SECRET}
grant_type: authorization_code
redirect_uri: https://www.google.com/

I've used the google homepage as a dummy redirect URI as we don't have any production Vend applications at this point. I'm not sure whether this is having any impact on the problem.

Authorization code:

https://www.google.com/?code=KWDZNSo67gnf82PHa805k_kLlcq7VYZHQeAhoK7J&domain_prefix=samplepsyirf01&user_id=0adfd74a-1555-11e9-fa42-466c8a5b0ef5&state=foostate&signature=2b575c8e2675845305d59040cef6e3f33b820839d8e8d4d073ebe2ac6167dca5

Vend API docs: https://docs.vendhq.com/reference#section-oauth-2-0