Can't create subdomain in primary domain root

As example I want create multiple language subdomains

en.domain.tld
fr.domain.tld
es.domain.tld

So I go to plesk and create a subdomain for domain.tld. In "The path to the website home directory." I clear input and submit and get this error "This required field is empty. You need to specify a value.".

Why I need to specify a value? I want it in the root folder of this domain. That start script(index.php) in root folder detect the name of the subdomain and set only a language flag. Actual...

Can't create subdomain in primary domain root