const options = {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  body: JSON.stringify({message: '<string>', signature: '<string>'})
};

fetch('https://api.gnosispay.com/api/v1/webhooks/subscribe/{partnerId}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

{
  "success": true,
  "subscriptionId": "<string>"
}