First you need to configure your server to accept any subdomain for your domain example.com
and redirect it to your virtual host that as well has to accept any subdomain. After that, you can use the following rule to rewrite that subdomain internally to a folder with the same name:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.
RewriteCond %{HTTP_HOST} ^([^/.]+)\.example\.com$
RewriteRule ^ %1%{REQUEST_URI} [L]