Mapping non-www to www primary?

I have a site” “domain.com” and it’s parked. I mapped this as a primary domain and all is working fine. My customer now wants only the “www.domain.com” version to show on their site. I edited the “domain.com” mapped domain and un-checked Primary. I then created a new domain: “www.domain.com” set it as primary and set the same site ID as “domain.com”. It’s not directly to WWW but not sure exactly why. Is this doing the same as an htaccess redirect?

Thanks!

https://www.ads-software.com/plugins/wordpress-mu-domain-mapping/