Server-side API (PHP / Node.js / Python)

<?php
$body = json_encode([
  "form_id"    => 1,
  "hash_field" => $_POST["email"],
  "fields"     => [
    ["field" => "email", "value" => $_POST["email"]],
    ["field" => "name",  "value" => $_POST["name"]],
    ["field" => "phone", "value" => $_POST["phone"]],
  ],
]);

$ch = curl_init("https://v2.1opd.ru/api/v2/create-agreement");
curl_setopt_array($ch, [
  CURLOPT_POST           => true,
  CURLOPT_POSTFIELDS     => $body,
  CURLOPT_HTTPHEADER     => [
    // sk_ + IP whitelist в ЛК ИЛИ sk_ + HMAC (см. вкладку)
    "API-KEY: sk_xxxxxxxxxxxxxxxxxxxx",
    "Content-Type: application/json",
  ],
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_TIMEOUT        => 5,
]);
$response = curl_exec($ch);
$data = json_decode($response, true);
curl_close($ch);

error_log("1ОПД hash: " . ($data["hash"] ?? "(error)"));