How to Redirect Apex Domain to www in Cloudflare

1. Set up DNS records

Create corresponding DNS A or CNAME records for the apex and subdomain.

2. Use Cloudflare DNS

Go to your registrar and add the Cloudflare Nameservers.

3. Create a redirect rule

Go to Redirect Rules and create a new rule.

When incoming requests match

Using the Expression Editor:
(lower(http.host) eq "example.com")

Then

Type: Dynamic
Expression: concat("https://","www.example.com",http.request.uri.path)
Status code: 301

Ref:
Cloudflare Docs, Redirect the apex domain to a subdomain