Just a little simple question ...

[joximu]

@edony44:
you can alter the file /etc/apache2/sites-enabled/master_oo.conf and deactivate the two lines with ServerName and ServerAlias and then make only one line

ServerName admin.yourdomain.tld

reload apache - finished.
Afterwards you should be able to let a user create yourdomain.tld (but he should not mak a subdomain named "admin"... of course... :-)