Subdomain to Domain with original URL kept

Question

I want to redirect a subdomain to a domain but keep the original subdomain URL.

The way my host works, is that /sub/subdomain/ is the standard subdomain root. Each subdomain has its own htaccess file /web/ is the domain root

This is what I put in the htaccess of subdomain

Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^subdomain.domain.com [NC]
RewriteRule ^/(.*)$ /$1 [L]

It does redirect to the domain, but doesn't keep the subdomain URL. What am I doing wrong?