How do you redirect a subdomain and mask the URL?

I'm able to redirect a subdomain to a different folder on the server but need to mask it as well so it doesn't show as "subdomain.domain.com/folder" but rather "subdomain.domain.com".

Here's what I'm using for the redirect:
RewriteCond %{REQUEST_URI} ^/folder1 [NC]
RewriteRule .* subdomain1.co.luxury [P,L]

Any solution or ideas are appreciated.
Thanks,
Eric

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