Get response from POST type method from restful API

How can I fetch the json response from the POST method? Currently I'm only able to fetch Status - 401 Unauthorized and StatusCode - 401

func postUrl(url string, byt []byte) (*http.Response, error) {
    tr := &http.Transport{
	    DisableCompression: true,
    }
    client := &http.Client{Transport: tr, Timeout: 10 * time.Second}
    req, err := http.NewRequest("POST", url, bytes.NewBuffer(byt))
    req.Header.Set("X-Custom-Header", "myvalue")
    req.Header.Set("Content-Type", "application/json")
    req.Header.Add("Authorization", "Basic "+basicAuth("username", "password"))
    resp, err := client.Do(req)
    return resp, err
  }

Above code produces the output:

{
  "errorMessages": [
    "You do not have the permission to see the specified issue.",
    "Login Required"
  ],
    "errors": {}
}

The way to read the response (if there is one) is the same regardless of what status you get.

defer resp.Body.Close()
body, err := ioutil.ReadAll(resp.Body)

As Frank said, regardless of the status code in the response you can simply read its body to use whatever content it has.

Particularly for the case of a JSON message, you have two options depending on whether you know the JSON message structure in advance (or want your code to depend on it).

If you know the structure and are ok with hard-coding it (plus you gain some type safety and better client code) you can have:

type ErrorResponse struct {
    Messages []string `json:"errorMessages"`
}

And then when you detect an error status code unmarshal the response body as that struct:

if resp.StatusCode % 100 != 2 {
    var error ErrorResponse
    err := json.Unmarshall(resp.Body, &error)
    // check err != nil ...
    // user error.ErrorMessages for whatever you want 
}

Alternatively if you don't want to depend on the JSON structure (to some degree) you can try to unmarshall it to a map[string]interface{} and try to use that in the generic way you think you can (generally not very useful).

This question is not related to http responses and http methods. Decode json string (wich is http response body in that case) with json decoder.

Simple example

(not directly related to your code snippet)

type Transition struct {
    Transition map[string]int
}

func main() {
    resp, err := postUrl(url, byt)
    if err != nil {
        log.Fatal(err)
    }

    var trans Transition
    decoder := json.NewDecoder(resp.Body)

    if err := decoder.Decode(&trans); err != nil {
        log.Fatal(err)
    }

    fmt.Printf("%+v\n", trans)
}