put this code in your DOCUMENT_ROOT/.htaccess
file:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^admins/?$ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L,NE]
That rule is only for /admins
URI. If you want to do this for all the URIs then use:
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L,NE]