RewriteEngine on
RewriteCond "%{HTTP_HOST}" "!^www\." [NC]
RewriteCond "%{HTTP_HOST}" "!^$"
RewriteRule "^/?(.*)" "http://www.%{HTTP_HOST}/subfolder/$1" [L,R,NE]
The above seems to work for me. See:
https://httpd.apache.org/docs/2.4/rewrite/remapping.html