curl --fail-with-body "$THORNODE_RPC_URL" \
  --header 'content-type: application/json' \
  --data '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "getFeeForMessage",
    "params": [
      "<BASE64_MESSAGE>",
      {
        "commitment": "processed"
      }
    ]
  }'

const body = {
  "jsonrpc": "2.0",
  "id": 1,
  "method": "getFeeForMessage",
  "params": [
    "<BASE64_MESSAGE>",
    {
      "commitment": "processed"
    }
  ]
};

const response = await fetch(process.env.THORNODE_RPC_URL, {
  method: "POST",
  headers: { "content-type": "application/json" },
  body: JSON.stringify(body),
});
const { result, error } = await response.json();
console.log(result ?? error);

{
  "jsonrpc": "2.0",
  "result": {
    "context": {
      "apiVersion": "4.1.2",
      "slot": 439749235
    },
    "value": 5000
  },
  "id": 1
}