Signature not match when try to request throught proxy url

[kittipongpad]

I use aws4 to sign my request to send to dynamodb service
It works when I use a simple dynamodb URL but its not work when I replace the host with my proxy URL
Do you have any idea on how we can sign requests and use proxy as hostname?
here's the example code

let payload = { "TableName":"simple-tablename" ....};

let option = {
      host:"${hostname}",
      service: 'dynamodb', 
      region: 'ap-southeast-1', 
      path: '/',
      method: 'POST',
      headers: {
         'Content-Type': 'application/x-amz-json-1.0',
         'X-Amz-Target': 'DynamoDB_20120810.UpdateItem'
       },
       body: JSON.stringify(payload) 
};
AWS4.sign(option, {
       accessKeyId: AWS.config.credentials?.accessKeyId,
       secretAccessKey: AWS.config.credentials?.secretAccessKey,
       sessionToken: AWS.config.credentials?.sessionToken
});
const req = https.request(option, (res) => { 
     console.log('statusCode: ' + res.statusCode);
     res.on('data', d => {
       const resObj = JSON.parse(d.toString());
       console.log(resObj);
     });
     });
     req.on('error', (error) => {
        console.error('error : ' + error);
     });
     req.write(data);
     req.end();
}

Its work when ${hostname} = 'dynamodb.ap-southeast-1.amazonaws.com' but not work when I use my proxy URL
Any help would be appreciated, perhaps I am missing something simple?