package main

import (
	"fmt"
	"strings"
	"net/http"
	"io/ioutil"
)

func main() {

	url := "https://api.sandbox.{id}.gr4vy.app/gift-card-services/{gift_card_service_id}"

	payload := strings.NewReader("{\n  \"display_name\": \"Qwikcilver UK\",\n  \"fields\": [\n    {\n      \"key\": \"private_key\",\n      \"value\": \"pk_26PHem9AhJZvU623DfE1x4sd\"\n    }\n  ],\n  \"active\": true\n}")

	req, _ := http.NewRequest("PUT", url, payload)

	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := ioutil.ReadAll(res.Body)

	fmt.Println(res)
	fmt.Println(string(body))

}

{
  "type": "gift-card-service",
  "id": "6c020bf3-179b-4f4f-858d-84e39e196e0f",
  "merchant_account_id": "default",
  "gift_card_service_definition_id": "qwikcilver-gift-card",
  "display_name": "Qwikcilver UK",
  "active": true,
  "fields": [
    {
      "key": "secret_key",
      "value": "sk_test_26PHem9AhJZvU623DfE1x4sd"
    }
  ],
  "created_at": "2012-12-12T10:53:43+00:00",
  "updated_at": "2012-12-12T10:53:43+00:00"
}