const url = 'https://api.gnosispay.com/api/v1/ibans/signing-message';
const options = {method: 'GET', headers: {Authorization: 'Bearer <token>'}, body: undefined};

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

{
  "data": {
    "message": "I hereby declare that I am the address owner."
  }
}