Similar to Amir Forsati's solution htaccess redirect to https://www but for variable domain name, I suggest:
RewriteEngine on
RewriteCond %{HTTPS} off [OR]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^(www\.)?(.+)$ [NC]
RewriteRule ^ https://www.%2%{REQUEST_URI} [R=301,L]