const url = 'https://api.gnosispay.com/api/v1/order/{orderId}/attach-transaction';
const options = {
  method: 'PUT',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: '{"transactionHash":"0x123abc..."}'
};

try {
  const response = await fetch(url, options);
  const data = await response.json();
  console.log(data);
} catch (error) {
  console.error(error);
}

{
  "ok": true
}