How to set a checkbox field to checked and read-only by default when sending documents using API?

curl -X 'POST' \
  'https://api.boldsign.com/v1/document/send' \
  -H 'accept: application/json' \
  -H 'X-API-KEY: {your API key}' \
  -H 'Content-Type: multipart/form-data' \
  -F 'Title="Sample Document"' \
  -F 'Signers={
  "name": "hanky",
  "emailAddress": "hankyWhites@gmail.com",
  "signerType": "Signer",
  "signerRole": "Signer",
  "formFields": [
    {
      "id": "CheckBox",
      "name": "CheckBox",
      "fieldType": "CheckBox",
      "value": "on",
      "pageNumber": 1,
      "bounds": {
        "x": 200,
        "y": 200,
        "width": 25,
        "height": 25
      },
      "isReadOnly": true
    }
  ],
  "locale": "EN"
}' \
  -F 'Files=@{your file}' \

var apiClient = new ApiClient("https://api.boldsign.com", "{Your API key}");
var documentClient = new DocumentClient(apiClient);

var documentFilePath = new DocumentFilePath
{
    ContentType = "application/pdf",
    FilePath = "{Your File path}"
};

var filesToUpload = new List<IDocumentFile>
{
    documentFilePath,
};

var checkboxField = new FormField(
    id: "CheckBox",
    type: FieldType.CheckBox,
    value: "on",
    isReadOnly: true,
    pageNumber: 1,
    bounds: new Rectangle(x: 200, y: 200, width: 25, height: 25));

var formFieldCollections = new List<FormField>()
{
    checkboxField
};

var signer = new DocumentSigner(
    signerName: "David",
    signerType: SignerType.Signer,
    signerEmail: "david@cubeflakes.com",
    formFields: formFieldCollections,
    locale: Locales.EN);

var documentSigners = new List<DocumentSigner>()
{
    signer
};

var sendForSign = new SendForSign()
{
    Signers = documentSigners,
    Title = "Sample Document",
    Files = filesToUpload
};
var documentCreated = documentClient.SendDocument(sendForSign);

import requests
import json

url = "https://api.boldsign.com/v1/document/send"

signer_data = {
    "name": "hanky",
    "emailAddress": "hankyWhites@gmail.com",
    "signerType": "Signer",
    "signerRole": "Signer",
    "formFields": [
    {
      "id": "CheckBox",
      "name": "CheckBox",
      "fieldType": "CheckBox",
      "value": "on",
      "pageNumber": 1,
      "bounds": {
        "x": 200,
        "y": 200,
        "width": 25,
        "height": 25
      },
      "isReadOnly": True
    }
    ],
    "locale": "EN"
}

payload = {
    'Signers': json.dumps(signer_data),
    'Title': "Sample Document"
}

files = [
    ('Files', ('{Your file name}', open('{Your file path}', 'rb'), 'application/pdf'))
]

headers = {
    'accept': 'application/json',
    'X-API-KEY': '{Your API key}'
}

response = requests.post(url, headers=headers, data=payload, files=files)

print(response.text)

const axios = require('axios');
const FormData = require('form-data');
const fs = require('fs');
let data = new FormData();
data.append('Signers', '{\r\n  "name": "hanky",\r\n  "emailAddress": "hankyWhites@gmail.com",\r\n  "signerType": "Signer",\r\n  "signerRole": "Signer",\r\n  "formFields": [\r\n    {\r\n      "id": "CheckBox",\r\n      "name": "CheckBox",\r\n      "fieldType": "CheckBox",\r\n      "value": "on",\r\n      "pageNumber": 1,\r\n      "bounds": {\r\n        "x": 200,\r\n        "y": 200,\r\n        "width": 25,\r\n        "height": 25\r\n      },\r\n "isReadOnly": true\r\n}\r\n  ],\r\n  "locale": "EN"\r\n}');
data.append('Files', fs.createReadStream('{Your file path}'));
data.append('Title', "Sample Document");

let config = {
  method: 'post',
  maxBodyLength: Infinity,
  url: 'https://api.boldsign.com/v1/document/send',
  headers: { 
    'accept': 'application/json', 
    'X-API-KEY': '{Your API key}', 
    ...data.getHeaders()
  },
  data : data
};

axios.request(config)
.then((response) => {
  console.log(JSON.stringify(response.data));
})
.catch((error) => {
  console.log(error);
});

<?php
require_once "vendor/autoload.php";
use GuzzleHttp\Client;
use GuzzleHttp\Psr7\Request;
use \GuzzleHttp\Psr7\Utils;
$client = new Client([
    'verify' => false, // Disable SSL verification
]);
$headers = [
  'accept' => 'application/json',
  'X-API-KEY' => '{Your API Key}'
];
$options = [
  'multipart' => [
    [
      'name' => 'Signers',
      'contents' => '{
        "name": "David",
        "emailAddress": "david@boldsign.dev",
        "signerType": "Signer",
        "formFields": [
            {
                "id": "CheckBox", 
                "name": "CheckBox", 
                "fieldType": "CheckBox",
                "value": "on",
                "pageNumber": 1, 
                "bounds": {
                  "x": 200, 
                  "y": 200, 
                  "width": 25, 
                  "height": 25 
                }, 
                "isRequired": true
            }
        ], 
        "locale": "EN"
      }'
    ],
    [
      'name' => 'Title',
      'contents' => 'Sample'
    ],
    [
      'name' => 'Files',
      'contents' => Utils::tryFopen('{Your File Path}', 'r'),
      'filename' => '{Your File name}',
      'headers'  => [
        'Content-Type' => 'application/pdf'
      ]
    ]
]];
$request = new Request('POST', 'https://api.boldsign.com/v1/document/send', $headers);
$res = $client->sendAsync($request, $options)->wait();
echo $res->getBody();
?>