Adding http headers in JsonRpcProvider of ethers.js

I like to add next http headers in JsonRpcProvider of ethers.js v6.x.

const headers = {
   'authorization': AUTH_VAL,
   'x-chain-id': '1001'

In v5.x version, ConnectionInfo object is accepted as the argument of constructor like next.

const connInfo = { url: URL, headers: headers };
const provider = new ethers.providers.JsonRpcProvider(connInfo);
const resp = await provider.send("klay_blockNumber", []);

A few links tell FetchRequest is the replacement of ConnectionInfo.
But next code lines result in error in the send() statement.

let fetchReq = new FetchRequest(url);
fetchReq.method = 'POST';
fetchReq.setHeader('authorization', AUTH_VAL);
fetchReq.setHeader('x-chain-id', '1001');
const resp = await provider.send('klay_blockNumber', []);

The error received:
{code: ‘SERVER_ERROR’, request: FetchRequest, response: FetchResponse, error: undefined, stack: ‘Error: server response 400 Bad Request (reque…commonjs/providers/provider-jsonrpc.js:239:40’, …}

I’d like to get some samples that set custom http headers and send Json RPC request with ethers.js v6.