not www to www

RewriteCond %{HTTP_HOST} !^www\.
RewriteRule (.*) http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Excluding ‘mysubdomain’

RewriteCond %{HTTP_HOST} !^(www|mysubdomain)\.
RewriteRule (.*) https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]