Making POST request in Go with formdata and authentication - go

Question

I'm currently trying to interface with an OAuth api with the example curl command curl -u {client_id}:{client_secret} -d grant_type=client_credentials https://us.battle.net/oauth/token. My current go file is:
package main
import (
"bytes"
"fmt"
"mime/multipart"
"net/http"
)
func checkErr(err error) bool {
if err != nil {
panic(err)
}
return true
}
func authcode(id string, secret string, cli http.Client) string {
//un(trace("authcode"))
var form bytes.Buffer
w := multipart.NewWriter(&form)
_, err := w.CreateFormField("grant_type=client_credentials")
checkErr(err)
req, err := http.NewRequest("POST", "https://us.battle.net/oauth/token", &form)
checkErr(err)
req.SetBasicAuth(id, secret)
resp, err := cli.Do(req)
checkErr(err)
defer resp.Body.Close()
json := make([]byte, 1024)
_, err = resp.Body.Read(json)
checkErr(err)
return string(json)
}
func main() {
//un(trace("main"))
const apiID string = "user"
const apiSecret string = "password"
apiClient := &http.Client{}
auth := authcode(apiID, apiSecret, *apiClient)
fmt.Printf("%s", auth)
}
When I run this I get a response of {"error":"invalid_request","error_description":"Missing grant type"}
For reference, the api flow states:
"To request access tokens, an application must make a POST request with the following multipart form data to the token URI: grant_type=client_credentials
The application must pass basic HTTP auth credentials using the client_id as the user and client_secret as the password."
and the expected response is a json string containing an access token, token type, expiration in seconds, and the scope of functions available with said token

Answer 1

From curl manual we have:
-d, --data <data>
(HTTP) Sends the specified data in a POST request to the HTTP server, in the same way that a browser does when a user has filled in an HTML form and
presses the submit button. This will cause curl to pass the data to the server using the content-type application/x-www-form-urlencoded. Compare to
-F, --form.
Note the content-type application/x-www-form-urlencoded part.
as opposed to:
-F, --form <name=content>
(HTTP SMTP IMAP) For HTTP protocol family, this lets curl emulate a filled-in form in which a user has pressed the submit button. This causes curl to
POST data using the Content-Type multipart/form-data according to RFC 2388.
Therefore based on your curl, mime/multipart is probably not what you're looking for and you should be using Client.PostForm, from the manual of which we have:
The Content-Type header is set to application/x-www-form-urlencoded. To set other headers, use NewRequest and Client.Do.