How can I convert PSPDFKIT API's python version in to golang

Question

i am using an API which named PSPDFKIT, this API can convert your local field in .pdf field .It has already write many language to demonstration,but i have a few proplem when converting in to golang. This is the version of Python which provided by PSPDFKIT: `

import requests
import json

instructions = {
  'parts': [
    {
      'html': 'index.html'
    }
  ]
}

response = requests.request(
  'POST',
  'https://api.pspdfkit.com/build',
  headers = {
    'Authorization': 'Bearer API key'
  },
  files = {
    'index.html': open('index.html', 'rb')
  },
  data = {
    'instructions': json.dumps(instructions)
  },
  stream = True
)

if response.ok:
  with open('result.pdf', 'wb') as fd:
    for chunk in response.iter_content(chunk_size=8096):
      fd.write(chunk)
else:
  print(response.text)
  exit()

`

and this is my code:

`

package main

import (
    "bytes"
    "encoding/json"
    "fmt"
    "io"
    "io/ioutil"
    "net/http"
    "os"
)

type instructions struct {
    Parts []struct {
        HTML string `json:"html"`
    } `json:"parts"`
}

func main() {
    instructions := `{
        'parts': [
          {
            'html': 'index.html'
          }
        ]
      }`
    instructionsBytes, err := json.Marshal(instructions)
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }

    file, err := os.Open("document.pdf")
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
    defer file.Close()

    response, _ := http.NewRequest("POST", "https://api.pspdfkit.com/build", bytes.NewBuffer(instructionsBytes))
    defer response.Body.Close()
    response.Header.Set("Authorization", "API key")
    response.Header.Set("Content-Type", "multipart/form-data")

    body, err := ioutil.ReadAll(response.Body)
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
    fmt.Println(string(body))
    os.Exit(1)

    result, err := os.Create("result.pdf")
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
    defer result.Close()

    _, err = io.Copy(result, response.Body)
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
}

` This API request the multipart/form-data in json:instructions and local field i guess,but i am not sure how to post multipart/form-data correctly,and i always received the empty .pdf field after running my code.

Can anyone help me? thanks a lot .

i wish to know how to post multipart/form-data correctly,also the right way to convert the code in golang,and finally it can produce the .pdf field that are not empty PDF.