How to add www to subdomain.domain.com on nginx ubuntu 14 digitalocean

How can I add WWW to my subdomain?

In my droplet I have domain.com and I already configure this domain to have WWW prefix using CNAME record. eg: www.domain.com

And now I have created a new subdomain using A record pointing to current droplet. Digital Ocean Control Panel

Then I pointed the subdomain to NginX folder NginX sites-available/default

After reloading NginX setting, my subdomain with WWW didn't work. Is there anything that i missed?

Digital Ocean, NginX, Ubuntu 14.04, PHP7