curl --request POST \
  --url https://api.example.com/signature-nonce \
  --header 'Content-Type: application/json' \
  --data '
{
  "leaf_index": "0x1"
}
'

const options = {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  body: JSON.stringify({leaf_index: '0x1'})
};

fetch('https://api.example.com/signature-nonce', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

{
  "signature_nonce": "0x0"
}