curl --request POST \
  --url https://api.relayapp.im/v1/messages/{message_id}/reactions \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "custom_emoji": "🔥",
  "target_part_id": "prt_01k1m4q9vn2r7t9b4c6qdh8xwy"
}
'

import requests

url = "https://api.relayapp.im/v1/messages/{message_id}/reactions"

payload = {
    "custom_emoji": "🔥",
    "target_part_id": "prt_01k1m4q9vn2r7t9b4c6qdh8xwy"
}
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({custom_emoji: '🔥', target_part_id: 'prt_01k1m4q9vn2r7t9b4c6qdh8xwy'})
};

fetch('https://api.relayapp.im/v1/messages/{message_id}/reactions', 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.relayapp.im/v1/messages/{message_id}/reactions",
  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([
    'custom_emoji' => '🔥',
    'target_part_id' => 'prt_01k1m4q9vn2r7t9b4c6qdh8xwy'
  ]),
  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.relayapp.im/v1/messages/{message_id}/reactions"

	payload := strings.NewReader("{\n  \"custom_emoji\": \"🔥\",\n  \"target_part_id\": \"prt_01k1m4q9vn2r7t9b4c6qdh8xwy\"\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://api.relayapp.im/v1/messages/{message_id}/reactions")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"custom_emoji\": \"🔥\",\n  \"target_part_id\": \"prt_01k1m4q9vn2r7t9b4c6qdh8xwy\"\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.relayapp.im/v1/messages/{message_id}/reactions")

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  \"custom_emoji\": \"🔥\",\n  \"target_part_id\": \"prt_01k1m4q9vn2r7t9b4c6qdh8xwy\"\n}"

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

{
  "reaction": {
    "message_id": "<string>",
    "target_part_id": "prt_01k1m4q9vn2r7t9b4c6qdh8xwy",
    "type": "love",
    "actor": {
      "kind": "user",
      "id": "usr_01JZU1F0BD"
    },
    "operation": "add",
    "changed": true,
    "custom_emoji": "❤️"
  }
}

{
  "error": {
    "code": "invalid_request",
    "message": "chat_id is required"
  }
}

{
  "error": {
    "code": "invalid_request",
    "message": "chat_id is required"
  }
}

{
  "error": {
    "code": "invalid_request",
    "message": "chat_id is required"
  }
}

{
  "error": {
    "code": "invalid_request",
    "message": "chat_id is required"
  }
}

{
  "error": {
    "code": "invalid_request",
    "message": "chat_id is required"
  }
}