I will use a domain name, quizzer.cloud, to demonstrate how to use custom domains for S3 static websites with CloudFormation template:

1. Start the template with the template version and a description (optional).
2. Define two parameters for accepting the root domain name and the sub-domain name, as follows:

Parameters:
  RootDomainName:
    Description: Domain name for your website (quizzer.cloud)
    Type: String
  SubDomainName:
    Description: Sub Domain name for your website (www.quizzer.cloud)
    Type: String

3. Define the root bucket resource, as follows:

Resources:
  RootBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Ref RootDomainName
      AccessControl: PublicRead
      WebsiteConfiguration:
        IndexDocument: index.html
        ErrorDocument: error.html

4. Define a bucket access policy that allows everyone to access the bucket's contents, as follows:

WebsitePublicAccessPolicy:
  Type: AWS::S3::BucketPolicy
  Properties:
    Bucket: !Ref RootBucket
    PolicyDocument:
      Statement:
        -
          Action:
            - "s3:GetObject"
          Effect: "Allow"
          Resource:
            Fn::Join:
              - ""
              -
                - "arn:aws:s3:::"
                - !Ref RootBucket
                - "/*"
          Principal: "*"

5. Create the redirect bucket, as follows:

WWWBucket:
  Type: AWS::S3::Bucket
  Properties:
    BucketName: !Ref SubDomainName
    AccessControl: BucketOwnerFullControl
    WebsiteConfiguration:
      RedirectAllRequestsTo:
        HostName: !Ref RootDomainName

6. Create two A records within the HostedZone, for the root domain and the sub-domain, as follows:

myDNS:
  Type: AWS::Route53::RecordSetGroup
  Properties:
    HostedZoneName: !Sub
      - ${DomainName}.
      - DomainName: !Ref RootDomainName
    Comment: Zone apex alias.
    RecordSets:
    -
      Name: !Ref RootDomainName
      Type: A
      AliasTarget:
        HostedZoneId: 'Z11RGJOFQNVJUP'
        DNSName: 's3-website.ap-south-1.amazonaws.com'
    -
      Name: !Ref SubDomainName
      Type: A
      AliasTarget:
        HostedZoneId: 'Z11RGJOFQNVJUP'
        DNSName: 's3-website.ap-south-1.amazonaws.com'

You can also use a CNAME record for sub-domains.

7. Add an Outputs section to return the URLs of the root domain and the sub-domain (this is optional):

Outputs:
  RootDomainURL:
    Value: !Sub
      - http://${DomainName}
      - DomainName: !Ref RootDomainName
    Description: URL for root domain
  SubDomainURL:
    Value: !Sub
      - http://${DomainName}
      - DomainName: !Ref SubDomainName
    Description: URL for redirect (sub) domain

8. Execute the preceding template by passing the parameters, as follows:

aws cloudformation create-stack 
    --stack-name mys3websitestack 
    --template-body file://resources/s3-static-website-cf-template.yml 
    --parameters ParameterKey=RootDomainName,ParameterValue=quizzer.cloud ParameterKey=SubDomainName,ParameterValue=www.quizzer.cloud 
    --region ap-south-1 
    --profile admin

You can check the status of the CloudFormation stack creation by using the describe-stacks command.

9. Once the stack creation has completed, you will need to upload the index.html and error.html files into the root bucket.

Finally, execute the root domain and the WWW sub-domain in a browser. 

You should see the same response from the root bucket in both cases, as follows: