To make your domain listen on www.example.com instead of example.com you have to do the following steps. (Make sure you replace example.com with your own domain)

1. Change the server_name variable inside your NGINX config. You can find this config file in this path (or you can use our UI to edit): /etc/nginx/sites-available/example.com
2. (Optional): If you have SSL enabled, you will have to edit the redirect too. Open up this file: /etc/nginx/ploi/example.com/before/ssl-redirect.conf and edit these lines:

server_name example.com

TO 

server_name www.example.com

And

return 301 https://example.com$request_uri;

TO 

return 301 https://www.$host$request_uri;

3. After this, save these files and restart NGINX.