not www to www
sh
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule (.*) http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
Excluding 'mysubdomain'
sh
RewriteCond %{HTTP_HOST} !^(www|mysubdomain)\.
RewriteRule (.*) https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]