It adds:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^online.pl$ [NC]
RewriteRule ^(.*)$ http://www.online.pl$1 [R=301,L]
While there should be just - this one works well:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]