package main

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

func main() {

	url := "https://api.sandbox.{id}.gr4vy.app/checkout/sessions/{session_id}/fields"

	payload := strings.NewReader("{\n  \"payment_method\": {\n    \"method\": \"card\",\n    \"number\": \"4111111111111111\",\n    \"expiration_date\": \"12/30\",\n    \"security_code\": \"123\"\n  }\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, _ := io.ReadAll(res.Body)

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

}

This response does not have an example.