URL change domain to sub domain

i have access to domain and server. also ftp and htaccess. i set new subdomain with name: shop.derbello.at

the subdomain should refer to extern domain: https://wa-markt.de/shop/derbello/ but client want to see his customers only his subdomain: shop.derbello.at and not the original site domain. here i have no access. possible to do? i try to slove it by htaccess method but its not work

RewriteEngine On
Options -Multiviews
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^login ./signIn.php`enter code here`