Submit variable in payload of golang http.NewRequest - json

Question

I'm learning Golang so please excuse what may seem to be a basic question. I have searched for a couple of hours for clues as to how I might achieve sending variable data in my JSON formatted API POST from my golang app, but not found a clue or solution yet. I know the answer will be my lack of syntax knowledge.
So the problem is with the 'lastcontact' field I'm trying to POST. I want to use my 'dt' variable that contains the current datetime.
package main
import (
"fmt"
"strings"
"net/http"
"io/ioutil"
"time"
)
func main() {
dt := time.Now()
url := "https://fakeapi.io/API/apiActions/update/"
method := "POST"
payload := strings.NewReader(`{
"name" : "Dumpty",
"saveconfig" : "true",
"lastcontact" : {dt}
}`)
client := &http.Client {
}
req, err := http.NewRequest(method, url, payload)
if err != nil {
fmt.Println(err)
return
}
req.Header.Add("api-key", "gjhgjhgjhg")
req.Header.Add("api-secret", "jhgjhgjhg")
req.Header.Add("Content-Type", "application/json; charset=UTF-8")
res, err := client.Do(req)
if err != nil {
fmt.Println(err)
return
}
defer res.Body.Close()
body, err := ioutil.ReadAll(res.Body)
if err != nil {
fmt.Println(err)
return
}
fmt.Println(string(body))
}

Answer 1

You can use a struct to store your response type if you know the fields beforehand. If you don't, you could use a map[string]interface{} to store arbitrary data.
Then using json.Marshal to convert it to a correctly formatted JSON response.
type myStruct struct {
Name string `json:"name"`
SaveConfig string `json:"saveconfig"`
LastContact time.Time `json:"lastcontact"`
}
dt := time.Now()
myData := myStruct{
Name: "Dumpty",
SaveConfig: "true",
LastContact: dt,
}
myBytes, err := json.Marshal(myData)
// bytes.NewBuffer returns *bytes.Buffer
// which implements the io.Reader interface
// that you need for your http.NewRequest call
payload := bytes.NewBuffer(myBytes)
Full playground example https://play.golang.org/p/Mc9UXT32Wi1