curl --request PUT \
  --url https://api.hubapi.com/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
[
  {
    "associationTypeId": 123
  }
]
'

import requests

url = "https://api.hubapi.com/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId}"

payload = [{ "associationTypeId": 123 }]
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.put(url, json=payload, headers=headers)

print(response.text)

const options = {
  method: 'PUT',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify([{associationTypeId: 123}])
};

fetch('https://api.hubapi.com/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId}', 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/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId}",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "PUT",
  CURLOPT_POSTFIELDS => json_encode([
    [
        'associationTypeId' => 123
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Bearer <token>",
    "Content-Type: application/json"
  ],
]);

$response = curl_exec($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/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId}"

	payload := strings.NewReader("[\n  {\n    \"associationTypeId\": 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(string(body))

}

HttpResponse<String> response = Unirest.put("https://api.hubapi.com/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId}")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("[\n  {\n    \"associationTypeId\": 123\n  }\n]")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.hubapi.com/crm/v4/objects/{objectType}/{objectId}/associations/{toObjectType}/{toObjectId}")

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

request = Net::HTTP::Put.new(url)
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "[\n  {\n    \"associationTypeId\": 123\n  }\n]"

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

{
  "fromObjectId": "<string>",
  "fromObjectTypeId": "<string>",
  "labels": [
    "<string>"
  ],
  "toObjectId": "<string>",
  "toObjectTypeId": "<string>"
}

{
  "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"
  }
}