curl --request POST \
  --url https://{instance}.my.salesforce.com/services/apexrest/v1/subscription/cancel \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "cancelRequests": [
    {
      "purchaseActivityId": "a0B5g00000PA1AAA",
      "cancellationType": "Cancel Renewal"
    }
  ]
}
'

import requests

url = "https://{instance}.my.salesforce.com/services/apexrest/v1/subscription/cancel"

payload = { "cancelRequests": [
        {
            "purchaseActivityId": "a0B5g00000PA1AAA",
            "cancellationType": "Cancel Renewal"
        }
    ] }
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    cancelRequests: [{purchaseActivityId: 'a0B5g00000PA1AAA', cancellationType: 'Cancel Renewal'}]
  })
};

fetch('https://{instance}.my.salesforce.com/services/apexrest/v1/subscription/cancel', 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://{instance}.my.salesforce.com/services/apexrest/v1/subscription/cancel",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    'cancelRequests' => [
        [
                'purchaseActivityId' => 'a0B5g00000PA1AAA',
                'cancellationType' => 'Cancel Renewal'
        ]
    ]
  ]),
  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://{instance}.my.salesforce.com/services/apexrest/v1/subscription/cancel"

	payload := strings.NewReader("{\n  \"cancelRequests\": [\n    {\n      \"purchaseActivityId\": \"a0B5g00000PA1AAA\",\n      \"cancellationType\": \"Cancel Renewal\"\n    }\n  ]\n}")

	req, _ := http.NewRequest("POST", 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.post("https://{instance}.my.salesforce.com/services/apexrest/v1/subscription/cancel")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"cancelRequests\": [\n    {\n      \"purchaseActivityId\": \"a0B5g00000PA1AAA\",\n      \"cancellationType\": \"Cancel Renewal\"\n    }\n  ]\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://{instance}.my.salesforce.com/services/apexrest/v1/subscription/cancel")

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

request = Net::HTTP::Post.new(url)
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"cancelRequests\": [\n    {\n      \"purchaseActivityId\": \"a0B5g00000PA1AAA\",\n      \"cancellationType\": \"Cancel Renewal\"\n    }\n  ]\n}"

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

{
  "success": true
}

{
  "success": false,
  "errors": [
    "businessUnitId is required"
  ]
}