For example https://example.com should be going to https://www.example.com
RewriteEngine On
RewriteCond %{SERVER_PORT} !=443
RewriteCond %{HTTP_HOST} ^(www\.)?example\.com$ [NC]
RewriteRule ^(.*)$ "https\:\/\/www\.example\.com\/$1" [R=301,L]
Note - you should change example.com to your own domain.