Make sure that the subdomain exists in dns too, otherwise it can not work as your webbrowser does not know that the subdomain exists.
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When I want to see the sub.domain.ext the server respond with the domain.ext
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Thats fine as the subdomain is an alias for the main website. If you want to get a different website, just create a new website for the subdomain instead of adding it to the domain.ext website as subdomain.