curl --request POST \
  --url https://api.hubapi.com/oauth/v3/token \
  --header 'Content-Type: application/x-www-form-urlencoded' \
  --data 'client_assertion=<string>' \
  --data 'client_assertion_type=<string>' \
  --data 'client_id=<string>' \
  --data 'client_secret=<string>' \
  --data 'code=<string>' \
  --data 'code_verifier=<string>' \
  --data 'redirect_uri=<string>' \
  --data 'refresh_token=<string>' \
  --data 'scope=<string>'

import requests

url = "https://api.hubapi.com/oauth/v3/token"

payload = {
    "client_assertion": "<string>",
    "client_assertion_type": "<string>",
    "client_id": "<string>",
    "client_secret": "<string>",
    "code": "<string>",
    "code_verifier": "<string>",
    "redirect_uri": "<string>",
    "refresh_token": "<string>",
    "scope": "<string>"
}
headers = {"Content-Type": "application/x-www-form-urlencoded"}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {'Content-Type': 'application/x-www-form-urlencoded'},
  body: new URLSearchParams({
    client_assertion: '<string>',
    client_assertion_type: '<string>',
    client_id: '<string>',
    client_secret: '<string>',
    code: '<string>',
    code_verifier: '<string>',
    redirect_uri: '<string>',
    refresh_token: '<string>',
    scope: '<string>'
  })
};

fetch('https://api.hubapi.com/oauth/v3/token', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.hubapi.com/oauth/v3/token",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => "client_assertion=%3Cstring%3E&client_assertion_type=%3Cstring%3E&client_id=%3Cstring%3E&client_secret=%3Cstring%3E&code=%3Cstring%3E&code_verifier=%3Cstring%3E&redirect_uri=%3Cstring%3E&refresh_token=%3Cstring%3E&scope=%3Cstring%3E",
  CURLOPT_HTTPHEADER => [
    "Content-Type: application/x-www-form-urlencoded"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

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

func main() {

	url := "https://api.hubapi.com/oauth/v3/token"

	payload := strings.NewReader("client_assertion=%3Cstring%3E&client_assertion_type=%3Cstring%3E&client_id=%3Cstring%3E&client_secret=%3Cstring%3E&code=%3Cstring%3E&code_verifier=%3Cstring%3E&redirect_uri=%3Cstring%3E&refresh_token=%3Cstring%3E&scope=%3Cstring%3E")

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

	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")

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

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

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://api.hubapi.com/oauth/v3/token")
  .header("Content-Type", "application/x-www-form-urlencoded")
  .body("client_assertion=%3Cstring%3E&client_assertion_type=%3Cstring%3E&client_id=%3Cstring%3E&client_secret=%3Cstring%3E&code=%3Cstring%3E&code_verifier=%3Cstring%3E&redirect_uri=%3Cstring%3E&refresh_token=%3Cstring%3E&scope=%3Cstring%3E")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.hubapi.com/oauth/v3/token")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Content-Type"] = 'application/x-www-form-urlencoded'
request.body = "client_assertion=%3Cstring%3E&client_assertion_type=%3Cstring%3E&client_id=%3Cstring%3E&client_secret=%3Cstring%3E&code=%3Cstring%3E&code_verifier=%3Cstring%3E&redirect_uri=%3Cstring%3E&refresh_token=%3Cstring%3E&scope=%3Cstring%3E"

response = http.request(request)
puts response.read_body

{
  "access_token": "<string>",
  "expires_in": 123,
  "refresh_token": "<string>",
  "token_type": "<string>",
  "token_use": "access_token",
  "hub_id": 123,
  "id_token": "<string>",
  "scopes": [
    "<string>"
  ],
  "user_id": 123
}

{
  "message": "Invalid input (details will vary based on the error)",
  "correlationId": "aeb5f871-7f07-4993-9211-075dc63e7cbf",
  "category": "VALIDATION_ERROR",
  "links": {
    "knowledge-base": "https://www.hubspot.com/products/service/knowledge-base"
  }
}