golang, post with json fails, but multipart succeed

Question

When I tried to send post with json, 401 error (unauthorized messages) showed. But with multipart, then succeeded.

In the following code,
I could get the token with only requestTokenByBuff which includes multipart.

Everytime I send post with json, I got failed. I tried to print json body, and there was no error. No header with body also made same result(401 error). I did not test http.Post because Header should be inputed.

Why does post with json fail?

type AuthConfig struct {
    ClientId     string `json:"client_id"`
    ClientSecret string `json:"client_secret"`
    Code         string `json:"data"`
    GrantType    string `json:"grant_type"`
}

var host string

func initAuthConfig(a *AuthConfig) {

    host = "https://xxxxxxx"

    a.ClientId = "xxxxxxx"
    a.ClientSecret = "xxxxxxx"
    a.Code = "xxxxx"
    a.GrantType = "xxxx"  

}

func initAuthConfig2() {

    host = "https://xxxxxxx"

}

func requestToken(auth *AuthConfig) {

    authbytes, _ := json.Marshal(auth)
    authbuff := bytes.NewBuffer(authbytes)

    client := &http.Client{}
    req, err := http.NewRequest("POST", host, authbuff)

    if err != nil {
        log.Fatalln(err)
    }
    req.Header.Add("User-Agent", "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.xxxxxx")
    req.Header.Add("Cookie", "SCOUTER=xxxxxxx") 
    req.Header.Add("Content-Type", "application/json")
    resp, err := client.Do(req)
    if err != nil {
        log.Fatalln(err)
    }
    defer resp.Body.Close()

    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        log.Fatalln(err)
    }
    data := string(body)
    fmt.Println(data)
}

func requestTokenByBuff(buff *bytes.Buffer) {

    writer := multipart.NewWriter(buff)
    _ = writer.WriteField("client_id", "xxxxxxxxxx")
    _ = writer.WriteField("client_secret", "xxxxxxxx")
    _ = writer.WriteField("data", "xxxxxx")
    _ = writer.WriteField("grant_type", "xxxxxxx")
    err := writer.Close()
    if err != nil {
        log.Fatalln(err)
        return
    }

    client := &http.Client{}
    req, err := http.NewRequest("POST", host, buff)

    if err != nil {
        log.Fatalln(err)
    }
    req.Header.Add("User-Agent", "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.xxxx")
    req.Header.Add("Cookie", "SCOUTER=xxxxxxxx")

    req.Header.Set("Content-Type", writer.FormDataContentType())
    resp, err := client.Do(req)
    if err != nil {
        log.Fatalln(err)
    }
    defer resp.Body.Close()

    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        log.Fatalln(err)
    }
    data := string(body)
    fmt.Println(data)
}

func main() {

    // config := new(AuthConfig)
    // initAuthConfig(config)
    // requestToken(config)

    buff := &bytes.Buffer{}
    initAuthConfig2()
    requestTokenByBuff(buff)
}